Apiman docs website & Antora playbook

This is where we generate our documentation using the wonderful Antora documentation website generator.

The actual documentation is stored in other repositories. This is where we maintain our Antora Playbook which pulls in the documentation sources from our various docs repos and generates the HTML for GitHub Pages.

To build the docs, you can run:

npm ci
# If you had a docs directory generated previously you want to clobber
rm -fr docs
# Use the locally installed version of Antora
npx antora generate --fetch --stacktrace antora-playbook.yml

Now you should see the website pop out in the docs directory, and you can open it in your browser of choice.