[FIRRTL][CAPI] Add documentation

[SpriteOvO]

Add documentation for all FIRRTL C-API public items.

[sequencer]

I think we can add more documentation for the unittest for firrtl.c? and provide a link to the panamaconverter usage from chisel.

[SpriteOvO]

I think we can add more documentation for the unittest for firrtl.c? and provide a link to the panamaconverter usage from chisel.

IMO, the test doesn't really need to be doc, they are all MLIR stuff, and these API are all already well documented.

I have added a link to Chisel example in the comment.