User-oriented Documentation for Firtool-lib C-API

A well-documented Firtool-lib C-API would be great for downstreams (e.g. `chisel`) references. There is a lack of such a thing at the moment.

So I opened the issue to discuss how this might be implemented. Any thoughts and opinions are welcome :)

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Things that need to be documented:

- All `populate-` functions.

- The effect of each option
(shared the same doc for both function setter and command-line opt)

Possible implementations:

- Write a Markdown file and place it under `./docs` in the git repository, then render as HTML and host it on https://circt.llvm.org/.

- Write code comments in the header files.

- Anything else?

Start by reviewing the firtool-lib C-API, its populate- functions, option setters, and corresponding command-line options. Decide whether the documentation belongs in a Markdown file under ./docs or in header-file comments; done means all populate- functions and each option's effect are documented for downstream users.

Written by the indexing model from the issue text.