[docs] add info on how to generate custom Node.js builds (elastic#168919

)

Co-authored-by: amyjtechwriter <[email protected]>

docs/developer/advanced/upgrading-nodejs.asciidoc

@@ -4,6 +4,12 @@
 {kib} requires a specific Node.js version to run.
 When running {kib} from source, you must have this version installed locally.
 
+=== Step 1: Generate custom Node.js builds
+
+Before making a PR to upgrade Node.js, we must first <<start-new-nodejs-build,generate the required custom Node.js builds>> matching the desired Node.js version.
+
+=== Step 2: Create PR to upgrade Node.js
+
 The required version of Node.js is listed in several different files throughout the {kib} source code.
 These files must be updated when upgrading Node.js:
 
@@ -16,8 +22,8 @@ These files must be updated when upgrading Node.js:
   - {kib-repo}blob/{branch}/package.json[`package.json`] - The version is specified in the `engines.node` field (if possible, also upgrade `@types/node` to match the new version, both under `devDependencies` and `resolutions`).
   - {kib-repo}blob/{branch}/WORKSPACE.bazel[`WORKSPACE.bazel`] - The version is specified in the `node_version` property.
     Besides this property, the list of files under `node_repositories` must be updated along with their respective SHA256 hashes.
-    These can be found on the https://nodejs.org[nodejs.org] website.
-    Example for Node.js v18.18.2: https://nodejs.org/dist/v18.18.2/SHASUMS256.txt.asc
+    These can be found in the `SHASUMS256.txt` file inside the public `kibana-custom-node-artifacts` GCP bucket.
+    Example for Node.js v18.18.2: https://storage.googleapis.com/kibana-custom-node-artifacts/node-glibc-217/dist/v18.18.2/SHASUMS256.txt[kibana-custom-node-artifacts/node-glibc-217/dist/v18.18.2/SHASUMS256.txt]
 
 See PR {kib-repo}pull/128123[#128123] for an example of how the Node.js version has been upgraded previously.
 
@@ -37,6 +43,13 @@ Due to Node.js 16 coming to an https://nodejs.org/en/blog/announcements/nodejs16
 To keep support for these older platforms, we're bundling the Linux distributable of {kib} with a https://github.com/elastic/kibana-custom-nodejs-builds[custom build of Node.js] with extended backwards compatibility.
 The only difference between the offical Node.js build and our custom build, is the version of `glibc` that it's compiled against.
 
+[[start-new-nodejs-build]]
+==== How to start a new build
+
+To generate a new custom Node.js build, https://buildkite.com/elastic/kibana-custom-node-dot-js-builds#new[start a new build] on our dedicated Buildkite pipeline (requires Elastic employee permissions).
+Give it a clear name (e.g. `Node 18.18.2`) and remember so set the custom `OVERRIDE_TARGET_VERSION` environment variable to the desired Node.js version - e.g. `OVERRIDE_TARGET_VERSION=18.18.2`.
+You find the "Environment Variables" field by expanding "Options >" in the "New Build" dialog.
+
 === Backporting
 
 The following rules are not set in stone.