Add GitHub Actions workflow for automated handbook generation

[Copilot]

Adds automation to regenerate handbook documentation on-demand and from wp-cli/wp-cli releases.

Changes

Workflow (.github/workflows/regenerate-handbook.yml)

• Manual trigger: workflow_dispatch for on-demand regeneration via Actions UI
• External trigger: repository_dispatch with regenerate-handbook event type for wp-cli/wp-cli to trigger on releases
• Installs WP-CLI nightly (using latest PHP version), non-bundled packages, runs wp handbook gen-all, commits/pushes changes

Documentation

• Updated README.md with workflow usage and information on how to trigger the workflow from wp-cli/wp-cli

To trigger from wp-cli/wp-cli, use:

- name: Trigger handbook regeneration
  uses: peter-evans/repository-dispatch@v2
  with:
    token: ${{ secrets.HANDBOOK_TRIGGER_TOKEN }}
    repository: wp-cli/handbook
    event-type: regenerate-handbook

Requires PAT with repo scope added as HANDBOOK_TRIGGER_TOKEN secret in wp-cli/wp-cli.

• Fixes Automate handbook generation #566

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Automate handbook generation</issue_title>
<issue_description>Would be nice if we could add a GitHub Actions workflow for this that could be triggered manually and even from the main framework repo. So whenever we release a new version, it would automatically trigger a handbook update.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Automate handbook generation #566

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[swissspidy]

# Example: Triggering Handbook Regeneration from wp-cli/wp-cli

To trigger the handbook regeneration from the main wp-cli/wp-cli repository when releasing a new version, add the following step to your release workflow:

```yaml
- name: Trigger handbook regeneration
  uses: peter-evans/repository-dispatch@v2
  with:
    token: ${{ secrets.HANDBOOK_TRIGGER_TOKEN }}
    repository: wp-cli/handbook
    event-type: regenerate-handbook
```

## Setup Requirements

1. Create a GitHub Personal Access Token (PAT) with `repo` scope
2. Add the PAT as a secret named `HANDBOOK_TRIGGER_TOKEN` in the wp-cli/wp-cli repository settings
3. The token must have write access to the wp-cli/handbook repository

## Alternative: Using GITHUB_TOKEN

If you prefer to use the built-in `GITHUB_TOKEN`, you can use:

```yaml
- name: Trigger handbook regeneration
  run: |
    curl -X POST \
      -H "Accept: application/vnd.github.v3+json" \
      -H "Authorization: token ${{ secrets.HANDBOOK_TRIGGER_TOKEN }}" \
      https://api.github.com/repos/wp-cli/handbook/dispatches \
      -d '{"event_type":"regenerate-handbook"}'
```

This will send a repository dispatch event that triggers the "Regenerate Handbook" workflow in the handbook repository.