[Docs] Setup automatic publishing webhook

The docs publisher can facilitate a scenario where book are automatically published when the corresponding branch is updated in their repository. This is typically achieved by making edits and submitting a pull request. Any emails listed in the commits that are submitted as part of the pull request will receive an email with a summary of the update process.

To set this up the following instructions are all that's needed:

GitLab

1. Go to https://lab.civicrm.org/extensions/civirules/settings/integrations
2. Set the URL to https://docs.civicrm.org/admin/listen
3. Set the Trigger to 'Push events' and 'Tag push events'.

This will mean that when docs content is updated the docs publisher will rebuild the published docs site.