Handle @service_tag

On #2264047: [meta] Document service definition tags → we decided to have a new documentation tag: @service_tag

The API module needs to:
- Parse them (they may have parameters so we need a syntax for those if we want them parsed specially)
- Display them on their own pages
- Link them like we did recently for services
- On the page for @defgroup service_tag, make a list of them

I think this last bit can be accomplished by essentially making it so that when one of these is detected, it's as if it had @ingroup service_tag in its documentation.