Move the supported table out of README

[ekohl]

This changes the workflow to have a rake task to create a database.md file and then render that into the generated documentation.

The release process is also enhanced to include the generated database.md file. This should allow sites like rubydoc.info correctly render it as well, though this hasn't been tested.

It also introduces a GitHub Action workflow to build GitHub Pages with the generated documentation. That means that the documentation should always be up to date and no manual steps are needed anymore. It also saves a lot of noise in the git log.

Please review carefully. Some notes:

• Have we set up GH Pages for voxpupuli/facterdb?
• Is the discoverability still all right?
• Doesn't this also build for all forks?