Automation of generation and PR submission for plugin API docs

Plugin API docs live into the documentation repository.
However they are generated based on the server repository contents.

There must be a plugin API docs generator (a github action on the mariadb-docs repository) that will:

• check out the server repository
• generate the documentation for it similarly to how MDEV-39806 does it.
• check out the mariadb-docs repository
• move the generated .md files directory into the local maradb-docs checkout.
• file a pull request for mariadb-docs if there's changes. It should look similar to https://github.com/mariadb-corporation/mariadb-docs/pull/535