API Plugin docs: add doxygen groups for the plugin interface and service headers

The generated Plugin API reference gets one page per @defgroup, and every @defgroup today is under include/mysql/psi/. None of the other headers has a group: the plugin*.h interface headers, the service_*.h headers and client_plugin.h. Their entries appear only on the combined api.md page: 656 of its 1,273 entries, including the audit, authentication and encryption plugin interfaces and all plugin services.

On the docs side we'd like to stop publishing api.md. It renders as 20.9 MB of HTML and repeats every group page (DOCS-6625). We can only do that once its entries have a group page, or they'd drop off the site.

Request: add @defgroup}}s that cover the non-PSI headers. One group per interface and one per service, or a single "Plugin Services" group, whichever suits you. Once the generated group pages cover everything {{api.md does, we'll stop installing api.md and repoint its links.