Focus Doxygen documentation

[jeffro256]

Right now Doxygen is documenting everything in the repo including submodules, everything in contrib, util, tests, etc. This bogs down the documentation to the point where it is very hard to navigate. I think it would be a good move to focus on documenting only the main C++ code which is specific to this repo.

Right now this means documenting src/ (without SUPERCOP) and contrib/epee/. After this commit, Doxygen went from running >6000 graphs to about 2200 graphs.

[jeffro256]

I'm also planning on adding a main page in a later commit

[LeoNero]

I agree with improving Doxygen, given that the first time I opened it, I found that there were to many information not related to the Monero codebase, which does not help beginners that much :)