[Enhancement] docs: add page about the config file

docs/usage/.pages

@@ -1,6 +1,7 @@
 title: Usage
 arrange:
   - commands.md
+  - configfile.md
   - kubeconfig.md
   - multiserver.md
   - guides

docs/usage/configfile.md

@@ -0,0 +1,120 @@
+# Config File
+
+## Introduction
+
+As of k3d v4.0.0, released in January 2021, k3d ships with configuration file support for the `k3d cluster create` command.
+This allows you to define all the things that you defined with CLI flags before in a nice and tidy YAML (as a Kubernetes user, we know you love it ;) ).
+
+!!! info "Syntax & Semantics"
+    The options defined in the config file are not 100% the same as the CLI flags.
+    This concerns naming and style/usage/structure, e.g.
+
+    - `--api-port` is split up into a field named `kubeAPI` that has 3 different "child fields" (`host`, `hostIP` and `hostPort`)
+    - k3d options are bundled in a scope named `options.k3d`, where `--no-rollback` is defined as `options.k3d.disableRollback`
+    - repeatable flags (like `--port`) are reflected as YAML lists
+
+## Required Fields
+
+As of the time of writing this documentation, the config file only **requires** you to define two fields:
+
+- `apiVersion` to match the version of the config file that you want to use (at this time it would be `apiVersion: k3d.io/v1alpha2`)
+- `kind` to define the kind of config file that you want to use (currently we only have the `Simple` config)
+
+So this would be the minimal config file, which configures absolutely nothing:
+
+```yaml
+apiVersion: k3d.io/v1alpha2
+kind: Simple
+```
+
+## Config Options
+
+The configuration options for k3d are continuously evolving and so is the config file (syntax) itself.
+Currently, the config file is still in an Alpha-State, meaning, that it is subject to change anytime (though we try to keep breaking changes low).
+
+!!! info "Validation via JSON-Schema"
+    k3d uses a [JSON-Schema](https://json-schema.org/) to describe the expected format and fields of the configuration file.
+    This schema is also used to [validate](https://github.com/xeipuuv/gojsonschema#validation) a user-given config file.
+    This JSON-Schema can be found in the specific config version sub-directory in the repository (e.g. [here for `v1alpha2`](https://github.com/rancher/k3d/blob/main/pkg/config/v1alpha2/schema.json)) and could be used to lookup supported fields or by linters to validate the config file, e.g. in yur code editor.
+
+### All Options: Example
+
+Since the config options and the config file are changing quite a bit, it's hard to keep track of all the supported config file settinggs, so here's an example showing all of them as of the time of writing:
+
+```yaml
+# k3d configuration file, saved as e.g. /home/me/myk3dcluster.yaml
+apiVersion: k3d.io/v1alpha2 # this will change in the future as we make everything more stable
+kind: Simple # internally, we also have a Cluster config, which is not yet available externally
+name: mycluster # name that you want to give to your cluster (will still be prefixed with `k3d-`)
+servers: 1 # same as `--servers 1`
+agents: 2 # same as `--agents 2`
+kubeAPI: # same as `--api-port myhost.my.domain:6445` (where the name would resolve to 127.0.0.1)
+  host: "myhost.my.domain" # important for the `server` setting in the kubeconfig
+  hostIP: "127.0.0.1" # where the Kubernetes API will be listening on
+  hostPort: "6445" # where the Kubernetes API listening port will be mapped to on your host system
+image: rancher/k3s:v1.20.4-k3s1 # same as `--image rancher/k3s:v1.20.4-k3s1`
+network: my-custom-net # same as `--network my-custom-net`
+token: superSecretToken # same as `--token superSecretToken`
+volumes: # repeatable flags are represented as YAML lists
+  - volume: /my/host/path:/path/in/node # same as `--volume '/my/host/path:/path/in/node@server[0];agent[*]'`
+    nodeFilters:
+      - server[0]
+      - agent[*]
+ports:
+  - port: 8080:80 # same as `--port '8080:80@loadbalancer'`
+    nodeFilters:
+      - loadbalancer
+labels:
+  - label: foo=bar # same as `--label 'foo=bar@agent[1]'`
+    nodeFilters:
+      - agent[1]
+env:
+  - envVar: bar=baz # same as `--env 'bar=baz@server[0]'`
+    nodeFilters:
+      - server[0]
+registries: # define how registries should be created or used
+  create: true # creates a default registry to be used with the cluster; same as `--registry-create`
+  use:
+    - k3d-myotherregistry:5000 # some other k3d-managed registry; same as `--registry-use 'k3d-myotherregistry:5000'`
+  config: | # define contents of the `registries.yaml` file (or reference a file); same as `--registry-config /path/to/config.yaml`
+    mirrors:
+      "my.company.registry":
+        endpoint:
+          - http://my.company.registry:5000
+options:
+  k3d: # k3d runtime settings
+    wait: true # wait for cluster to be usable before returining; same as `--wait` (default: true)
+    timeout: "60s" # wait timeout before aborting; same as `--timeout 60s`
+    disableLoadbalancer: false # same as `--no-lb`
+    disableImageVolume: false # same as `--no-image-volume`
+    disableRollback: false # same as `--no-Rollback`
+    disableHostIPInjection: false # same as `--no-hostip`
+  k3s: # options passed on to K3s itself
+    extraServerArgs: # additional arguments passed to the `k3s server` command; same as `--k3s-server-arg`
+      - --tls-san=my.host.domain
+    extraAgentArgs: [] # addditional arguments passed to the `k3s agent` command; same as `--k3s-agent-arg`
+  kubeconfig:
+    updateDefaultKubeconfig: true # add new cluster to your default Kubeconfig; same as `--kubeconfig-update-default` (default: true)
+    switchCurrentContext: true # also set current-context to the new cluster's context; same as `--kubeconfig-switch-context` (default: true)
+  runtime: # runtime (docker) specific options
+    gpuRequest: all # same as `--gpus all`
+
+```
+
+## Config File vs. CLI Flags
+
+k3d uses [`Cobra`](https://github.com/spf13/cobra) and [`Viper`](https://github.com/spf13/viper) for CLI and general config handling respectively.
+This automatically introduces a "config option order of priority" ([precedence order](https://github.com/spf13/viper#why-viper)):
+
+!!! info "Config Precedence Order"
+    Source: [spf13/viper#why-viper](https://github.com/spf13/viper#why-viper)
+
+    Internal Setting > **CLI Flag** > Environment Variable > **Config File** > (k/v store >) Defaults
+
+This means, that you can define e.g. a "base configuration file" with settings that you share across different clusters and override only the fields that differ between those clusters in your CLI flags/arguments.
+For example, you use the same config file to create three clusters which only have different names and `kubeAPI` (`--api-port`) settings.
+
+## References
+
+- k3d demo repository: <https://github.com/iwilltry42/k3d-demo/blob/main/README.md#config-file-support>
+- SUSE Blog: <https://www.suse.com/c/introduction-k3d-run-k3s-docker-src/> (Search fo `The “Configuration as Code” Way`)