docgen: fixes Fixes #3333, #3334, #3408

[ManuLinares]

Fixes #3333, #3334, #3408

• Emit attrdef declarations and attached doc comments in JSON output.
• Include doc comments on alias declarations in JSON output.
• Render contract @return documentation in docs.html symbol details.
• Suppress empty docs, members, and name JSON fields. (make it slimmer basically)
• Refactor docgen.c JSON emission using JsonEmitter API. This was needed for my sanity.
• and docs.html general cleanup

I've also:

• Excluded compiler_rt from --emit-stdlib=no on docgen.