[TF-M] [RFC] Doxygen usage

Hi,

As mentioned in Tech Forum, call for feedbacks for doxygen usage:

- Anyone is checking doxygen documents? - Which API are you checking with doxygen documents? The service APIs, or even some internal static API for SPM? - When you are reading code, will you check the docs or you just open them in editors and then read it?

The background is we put many efforts on docs, it costs effort to maintain the doxygen format comments for all sources even the concept is covered in the docs folder, we are trying to find a balance to help mitigate the effort spent on internal logics (such as those static APIs inside SPM fewer people would update). For the interfaces API, doxygen would be always there so user can find them easily.

Thanks

/Ken