API Plugin docs: Make moxygen doc be a good GitBook citizen

No H5 and H6 cause GitBook can't process these.

Please change any H5 and H6 headings to H3 and H4 in the Doxygen/Moxygen that creates the API Plugin docs for https://mariadb.com/docs/server/reference/plugins/api-plugin.

344 in-page links in the generated pages don't resolve, and I confirmed it against the rendered GitBook preview rather than just trusting the checker: id="m_file", id="m_psi" and the rest simply aren't in the HTML, and the page contains zero <h5>/<h6> elements.

Alternatives identified to fix this are:

Patch moxygen

Submit a pull request to moxygen to add an extra option to further restrict the generated headings levels

Implement a post-processing sed filter

Add a step to the .md generation script to remove the 5- and 6- # signs and replace these with h4s.

Drop `--anchors` from the moxygen command line

GitBook can'd deal with these.