docs: add initial experimental vitest testing guide (angular#61483)

An initial guide to use the experimental unit testing system with the
vitest runner has been added. This includes setup instructions for switching
from the karma/jasmine based system.
Additional content for configuration and potential migration of existing
projects may be added in future changes.

PR Close angular#61483

Author: clydin

adev/src/app/sub-navigation-data.ts

@@ -501,6 +501,11 @@ const DOCS_SUB_NAVIGATION_DATA: NavigationItem[] = [
             path: 'guide/testing/utility-apis',
             contentPath: 'guide/testing/utility-apis',
           },
+          {
+            label: 'Experimental unit testing integration',
+            path: 'guide/testing/unit-tests',
+            contentPath: 'guide/testing/experimental-unit-test',
+          },
           {
             label: 'Component harnesses overview',
             path: 'guide/testing/component-harnesses-overview',

adev/src/content/guide/testing/experimental-unit-test.md

@@ -0,0 +1,65 @@
+# Experimental unit testing system
+
+The Angular CLI provides an experimental unit test system that can use [Vitest](https://vitest.dev/) as a test runner.
+
+IMPORTANT: This experimental unit testing system requires the use of the `application` build system.
+The `application` build system is the default for all newly created projects.
+
+## Set up testing
+
+The Angular CLI includes the test system within a new project but must be configured before it can be used.
+
+The project you create with the CLI is setup to use the `karma` test system by default.
+To change to the experimental unit test system, update the `test` target as follows:
+
+<docs-code language="json">
+"test": {
+  "builder": "@angular/build:unit-test",
+  "options": {
+    "tsConfig": "tsconfig.spec.json",
+    "runner": "vitest",
+    "buildTarget": "::development",
+  }
+}
+</docs-code>
+
+The `buildTarget` operates similarly to the option available to the development server.
+The `build` target configures build options for the tests.
+If the `development` build configuration is missing for a project or you need
+different options for testing, you can create and use a `testing` or similarly named build configuration.
+
+To execute the application's testing, just run the [`ng test`](cli/test) CLI command as before:
+
+<docs-code language="shell">
+
+ng test
+
+</docs-code>
+
+The `ng test` command builds the application in *watch mode*, and launches the configured runner.
+
+The console output looks like below:
+
+<docs-code language="shell">
+ ✓ spec-app-app.spec.js (2 tests) 31ms
+   ✓ App > should create the app 22ms
+   ✓ App > should render title 8ms
+
+ Test Files  1 passed (1)
+      Tests  2 passed (2)
+   Start at  14:24:15
+   Duration  1.16s (transform 628ms, setup 703ms, collect 64ms, tests 31ms, environment 188ms, prepare 33ms)
+</docs-code>
+
+Watch mode is enabled by default when using an interactive terminal and not running on CI.
+
+## Configuration
+
+The Angular CLI takes care of the Vitest configuration for you. It constructs the full configuration in memory, based on options specified in the `angular.json` file.
+Directly customizing the underlying test runner configuration is currently not supported.
+
+## Bug reports
+
+Report issues and feature requests on [GitHub](https://github.com/angular/angular-cli/issues).
+
+Please provide a minimal reproduction where possible to aid the team in addressing issues.