Generating libbpf API documentation

[Grant Seltzer Richman <grantseltzer@gmail.com>]

Hi all,

I have been experimenting with ways to contribute documentation to
libbpf to make it easier for developers of bpf projects to use it.
With the goal of making a documentation site that is easy to
maintain/generate I found Doxygen (many of you may have experience
with it, I did not). I set up a CI/CD workflow using github actions
that runs doxygen on the libbpf mirror hosted there, and hosts the
produced HTML using netlify. You can find the currently hosted version
of it at https://libbpf-docs.netlify.app (I would gladly donate a real
domain name for this purpose). The docs generation workflow is in my
github repo here: https://github.com/grantseltzer/libbpf-docs

In order to make this work all we would need is to format comments
above functions we want to document. Doxygen requires that the comment
just be in a block that starts with `/**`. I don't think doxygen
specific directives should be committed to code but I think this is a
fine convention to follow. Other doxygen directives (i.e. having
`@file` in every file) can be faked using a step I have in the github
actions.

What does everyone think? Can we agree on this convention and start
contributing documentation in this way? Any pitfalls to doxygen I'm
not familiar with?

Thanks!