Add documentation for how codegen works in the sdk (#23742)

* Add documentation for how codegen works in the sdk

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docs/code_generation.md

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+# Code generation
+
+## Code generation inputs (`*.zap` files)
+
+Matter code relies on code generation for cluster-specific data types and
+callbacks. Generally this is split into:
+
+-   Data serialization for structures/lists/commands. This applies to both
+    client-side and server-side structures and objects
+-   Callback setup using the Ember-based framework. This generally applies to
+    server-side processing and the code generation defines what processing needs
+    to be done when a specific command is received or an attribute is read and
+    what memory should be allocated for storing cluster attributes
+
+Code generation depends on the clusters that are needed by an application. Every
+application configures the specific set of endpoints and clusters it needs based
+on the device type it supports. The selection of the supported clusters and
+attributes (as optional attributes may be omitted to save memory) is generally
+stored in `*.zap` files.
+
+The selection of enabled clusters and files is done using
+[ZAP](https://github.com/project-chip/zap). You can download a recent release of
+zap from its [releases page](https://github.com/project-chip/zap/releases). It
+is recommended to download a release that is in sync with the currently in use
+version by the SDK (see `integrations/docker/images/chip-build/Dockerfile` and
+check the `ZAP_VERSION` setting).
+
+Beyond basic zap file selection, there are also `.json` zap settings that define
+additional cluster info: source XML files, sdk-access methods and data types.
+There are only two such files currently in use:
+
+-   `src/app/zap-templates/zcl/zcl.json` is the **default** one
+-   `src/app/zap-templates/zcl/zcl-with-test-extensions.json` is used by
+    `all-clusters-app` to show how a cluster extension may be configured with
+    minimal changes from `zcl.json` (but it is different)
+
+### Installing zap and environment variables
+
+Matter scripts may need to invoke `zap-cli` (for code generation) or `zap` (to
+start the UI tool). For this, scrips need to know where to find the commands. In
+the following order, the scripts process these environment variables:
+
+-   if `$ZAP_DEVELOPMENT_PATH` is set, code assumes you are running zap from
+    source. Use this if you develop zap. Zap has to be bootstrapped (generally
+    `npm ci` but check zap documentation for this. Some scripts have a
+    `--run-bootstrap` command line argument to do this for you)
+
+-   if `$ZAP_INSTALL_PATH` is set, code assumes that `zap` or `zap-cli` is
+    available in the given path. This is generally an unpacked release.
+
+-   otherwise, scripts will assume `zap`/`zap-cli` is in `$PATH`
+
+### Using a UI to edit `.zap` files
+
+Generally you need to invoke zap with appropriate zcl and generate arguments.
+Most of code generation is app specific, so you generally want something of the
+form
+`--gen src/app/zap-templates/app-templates.json --zcl $ZCL_JSON_FILE $ZAP_FILE_TO_EDIT`
+
+Since this is tedious to type, the SDK provides a
+`scripts/tools/zap/run_zaptool.sh` script to automate this:
+
+```bash
+# Ensure zap is in $PATH or set $ZAP_INSTALL_PATH or $ZAP_DEVELOPMENT_PATH
+./scripts/tools/zap/run_zaptool.sh examples/lighting-app/lighting-common/lighting-app.zap
+```
+
+### Human-readable code generation inputs (`*.matter`)
+
+`.zap` files are large json files that are generally not human readable. As a
+result, the Matter SDK also keeps an equivalent `*.matter` file along side
+`.zap` files that contain the same data as `.zap` files, targeted specifically
+for matter:
+
+-   They are designed to be human readable, looking like a IDL (think protobuf
+    or android `aidl`, thrift idl etc.)
+
+-   We strive to make them contain only Matter-specific data (`.zap` files
+    contain more generic data and is designed to be ZigBee backwards compatible)
+
+Currently `.matter` files are generated from `.zap` files during the application
+specific codegen.
+
+### `*.matter` parsing and codegen
+
+`*.matter` files are both human and machine readable. Code that can process
+these files is available at `scripts/idl` and `scripts/codegen.py`. You can read
+the [scripts/idl/README.md](../scripts/idl/README.md) for details of how things
+work.
+
+`scripts/codegen.py` can generate various outputs based on an input `*.matter`
+file.
+
+The split between `.zap` and `.matter` currently exists as an experiment of code
+generation technologies. Currently `.matter`-based Python code generation:
+
+-   has fewer third party dependencies than `zap`, which installs a significant
+    number of `npm` packages.
+-   runs significantly faster than zap
+-   offers more flexible code generation (can generate multiple files per
+    cluster for example, without which some compiles would run out of RAM on
+    large compilations)
+-   has a more flexible templating language
+-   has human readable (and potentially editable) input
+-   is more easily provable deterministic (`zap` uses an underlying sqlite
+    database and some legacy assumptions from zigbee have historically caused
+    non-determinism)
+-   uses a synchronous processing model which is potentially easier to develop
+    for
+-   has lower complexity, is unit tested and uses typing extensively
+
+Ideally, the project would be to have a single code generation method in the
+long term that has all the benefits and none of the drawbacks. We are not there
+yet, however we likely want:
+
+-   Flexible codegen (we will need to split output by clusters or other rules)
+-   Human-readable inputs that enable code reviews and audits
+-   Rules that a script can validate based on CSA data model (ensure mandatory
+    attribute settings are followed, ensure proper device type adherence, ensure
+    correct cluster and data type definitions)
+-   Easy to maintain and develop for chosen languages/templates/codegen in
+    general
+
+## Code generation outputs and templates
+
+Code that is generated:
+
+-   **Application-specific**:
+
+    -   ZAP generation is based on `.zap` files in `examples/` and generates
+        server-side processing data: what cluster callbacks to set up, what RAM
+        to reserve for attribute storage etc.
+
+    -   `Codegen.py` will also generate a subset of application-specific files
+
+-   **Automated tests**: embedded client-side tools (`chip-tool` and
+    `darwin-framework-tool`) generate test-definition data. Each use their own
+    `examples/${TOOL}/templates/tests/templates.json` to drive what gets
+    generated.
+
+-   **Controller clusters** target: the file
+    `src/controller/data_model/controller-clusters.zap` contains a set of
+    cluster selections to which all applications would potentially have access.
+    These are generally used as `all clusters selection` and the intent is to
+    allow any application to access any cluster as a `client side`.
+
+    Client/controllers will codegen based on this, like **tools**, **tests**,
+    **java**, **python** etc.
+
+## Running codegen
+
+### ZAP file generation
+
+Generating all possible code (all categories above) using zap tool can be done
+via:
+
+```bash
+./scripts/tools/zap_regen_all.py
+```
+
+This can be slow (several minutes). The regen tool allows selection of only
+tests so that yaml test development goes faster.
+
+```bash
+./scripts/tools/zap_regen_all.py --type tests
+./scripts/tools/zap_regen_all.py --type tests --tests chip-tool
+```
+
+Additionally, individual code regeneration can be done using
+`./scripts/tools/zap/generate.py`:
+
+```bash
+/scripts/tools/zap/generate.py examples/bridge-app/bridge-common/bridge-app.zap -o zzz_generated/bridge-app/zap-g
+enerated
+```
+
+### `*.matter` code generation
+
+Currently `*.matter` code generation is done at compile time.
+
+Rules for how `codegen.py` is invoked and how includes/sources are set are
+defined at:
+
+-   `src/app/chip_data_model.cmake`
+-   `build/chip/esp32/esp32_codegen.cmake` (support for 2-pass cmake builds used
+    by the Espressif `idf.py` build system)
+-   `src/app/chip_data_model.gni`