Improve doxygen documentation

I think it is a good first issue as a newcomer you want to learn about the API. Improving the documentation is exactly that :-)

When looking at https://api.libssh.org/master/topics.html it already shows several issues.

There are categories which are out of the main category. Also the descriptions are empty for most categories missing @brief elements. It would also look a bit nicer with emojis ;-)

Once this is done each category would need to be check if functions are in the right category and what documentation is missing. If documentation is missing it would be nice to have a bug report or merge request for it.