Doxygen documentation

[mlund]

Doxygen is a de facto standard for documenting C/C++ projects and has several benefits:

1. it's picked up by most IDE's to give help in functions, defines, etc.
2. it can be used to output HTML, LaTeX, and XML
3. XML output can be further parsed to e.g. the MEGA65 User Guide

This PR adds the following to mega65-libc:

• make doc build target that creates HTML, LaTeX, and XML documentation of the API
• make guide runs a python script to extract Doxygen tags to MEGA65 User Guide LaTeX source. This could replace the ad-hoc \m65lib tags currently used and if so, is now available for all functions in libc (not just conio.h as of now).
• Pre-commit configuration for clang-format and ruff
• README.md updated with instructions for building documentation
• Add a contributer check-list that will show up on new PR's on github
• This PR contains no code changes

Note

This is a second version of this PR do force an update of the diff after #39 was merged.