docs: add ngcc postinstall migration doc (#33328)

PR Close #33328

.github/CODEOWNERS

@@ -887,6 +887,7 @@ testing/**                                                      @angular/fw-test
 /aio/content/guide/migration-injectable.md                      @angular/fw-docs-packaging @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
 /aio/content/guide/migration-localize.md                        @angular/fw-docs-packaging @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
 /aio/content/guide/migration-module-with-providers.md           @angular/fw-docs-packaging @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
+/aio/content/guide/migration-ngcc.md                            @angular/fw-docs-packaging @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
 
 
 # ================================================

aio/content/guide/deprecations.md

@@ -366,6 +366,11 @@ See the [dedicated migration guide for adding missing `@Injectable` decorators](
 ### Migrating `ModuleWithProviders`
 
  See the [dedicated migration guide for `ModuleWithProviders`](guide/migration-module-with-providers).
+{@a ngcc-migration}
+### Migrating to `ngcc` npm `postinstall` script
+
+ See the [dedicated migration guide for `ngcc` npm `postinstall` script](guide/migration-ngcc).
+
 
 
 {@a removed}

aio/content/guide/migration-ngcc.md

@@ -0,0 +1,79 @@
+# Migration to `ngcc` npm `postinstall` script
+
+## What does this schematic do?
+
+This schematic adds an [Angular compatibility compiler](guide/ngcc), or `ngcc`, invocation to npm/yarn's `postinstall` script in the `package.json` of an Angular CLI workspace.
+This script is invoked after each execution of `npm install` and modifies `node_modules` by converting any found Angular libraries to a format that is compatible with Angular version 9.
+
+An example diff might look like the following:
+
+**Before:**
+
+```json
+  "scripts": {
+    "ng": "ng",
+    "start": "ng serve",
+    "build": "ng build",
+    "test": "ng test",
+    "lint": "ng lint",
+    "e2e": "ng e2e"
+  },
+```
+
+**After:**
+
+```json
+  "scripts": {
+    "ng": "ng",
+    "start": "ng serve",
+    "build": "ng build",
+    "test": "ng test",
+    "lint": "ng lint",
+    "e2e": "ng e2e",
+    "postinstall": "ngcc --properties es2015 browser module main --first-only --create-ivy-entry-points"
+  },
+```
+
+If the `package.json` already contains a `postinstall` script, then the `ngcc` invocation will be prepended to the current command:
+
+**Before:**
+```json
+  "scripts": {
+    ...
+    "postinstall": "some-command"
+  },
+```
+
+**After:**
+```json
+  "scripts": {
+    ...
+    "postinstall": "ngcc --properties es2015 browser module main --first-only --create-ivy-entry-points && some-command"
+  },
+```
+
+
+## Why is this migration necessary?
+
+This migration is a build performance optimization that enables `ngcc` to parallelize the compilation of npm libraries.
+An application build performed via CLI's `ng build` should succeed regardless of this `postinstall` script being installed, because the CLI has `ngcc` built-in.
+However, this built-in `ngcc` can't parallelize the compilation of multiple libraries, and therefore often takes considerably longer to run.
+
+
+## Can I customize the `ngcc` options in the `postinstall` script?
+
+By default the `postinstall` script invokes `ngcc` with options to compile only the most commonly needed library formats.
+For some projects, especially those that depend on the CommonJS distribution of Angular (for example, Angular Universal apps), it might be beneficial to modify the `postinstall` script to also compile the CommonJS distribution of the library:
+
+```json
+  "scripts": {
+    ...
+    "postinstall": "ngcc --properties es2015 browser module main --first-only --create-ivy-entry-points && ngcc --properties main --create-ivy-entry-points"
+  },
+```
+
+For the full set of options run `npx ngcc --help` or `yarn ngcc --help`.
+
+## Will libraries compiled with `ngcc` still be compatible with Angular version 8?
+
+Yes, the migration causes `ngcc` to be invoked with the `--create-ivy-entry-points` flag, which ensures that the `ngcc` compilation is non-destructive, so the same `node_modules` can be used with Angular version 8 and version 9.