Add doxygen usage guide to our contributing documentation

[dbkinder]

Early versions of the Zephyr technical docs included contributing information for using doxygen comments for API documentation but was removed in later versions. Bring back a practical doxygen usage guide with our project conventions on documenting API interfaces with doxygen comments, how to use Sphinx directives to add the API material to the published documentation set, and known issues and workarounds (e.g., unnamed nested structs/unions).

### Things that should be documented
- [ ] How to document internal/hidden API
- [ ] Zephyr-specific commands/aliases such as `@kconfig`
- [ ] How to check API coverage for the documentation
- [ ] How to mount doxygen documentation in Sphinx
- [ ] How to document driver API (related to #63286)
- [ ] Use `@file` tag in each documented file
- [ ] When to use `@defgroup` vs. `@name`

[dbkinder]

Note how some APIs depend on KConfig settings, as described in #21727, and should be noted in the API doxygen comments.