fix the structure of the plugin MD file

https://jira.mariadb.org/browse/MDEV-39718 adds the infrastructure to produce the plugin .md file.

But the result is just an enumeration of the existing structures, functions and defines from the plugin API header files.

This is about generating the skeleton of a proper plugin document, namely:

• a main page that readers should start with
• a plugin type sub-page with the relevant amount of sub-pages for each plugin type
• a plugin services sub-page with the relevant amount of sub-pages for each plugin service