contributing: add section about documentation conventions

Document the existing conventions.

Use `mystruct.foo` instead of `mystruct::foo` because `::` is pretty
alien to C.

Instead of backticks, use a different format to reference declarations
in our docs:

    See foo().
    See struct foo.
    See union bar.
    See enum baz.
    See typedef meh.

This is inspired by the kernel's documentation style [1]. This format
has the upside of being pretty natural to write and read, and can be
automatically processed by documentation generators.

[1]: https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html#cross-referencing-from-restructuredtext

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