Early developer documentation for C bindings

This:

- Moves documentation on C bindings to its own page, moving the existing style guide
- Changes the existing style guide to use C-style `/* comments */` instead of C++-style `// comments`.
- Add a note to the style guide about using that comment style.
- Add some info on adding conditionally available APIs such as constants, functions...

Hopefully the start of a lot of documentation on how to write some C bindings :)