doxygen should use relevant "module" defs to organize the docs

[wmamills]

Zephyr uses doxygengroup to integrate doxygen content in place in the sphinx docs.
We plan to do this as well.

libmetal already has well defined "groups" aka "modules" that organize the content into API sub groups.

open-amp should do the same. This can be done and tested before Sphinx integration by just browsing the "modules" pull down menu in the default doxygen content.

[github-actions]

This issue has been marked as a stale issue because it has been open (more than) 45 days with no activity.

[wmamills]

This is not done

[wmamills]

Please compare: https://openamp.readthedocs.io/en/latest/doxygen/openamp/modules.html
And https://openamp.readthedocs.io/en/latest/doxygen/libmetal/modules.html

libmetal is much better organized and easier to understand.

[github-actions]

This issue has been marked as a stale issue because it has been open (more than) 45 days with no activity.