Documentation/process for adding new callbacks

[AndrewFasano]

It used to be fairly straightforward to add a new callback, for example: https://github.com/panda-re/panda/commit/95fb1a9d0b189168d89ebb19765c35f1a9847756. We would:

• Add to cb-defs.h enum + docs and signature
• Add 1-line macro to cb-support.c
• Execute the self-updating cb-support.h script and pypanda's create_datatypes.py scripts

Since merging #1105 this process has gotten a bit more complex and I couldn't find any documentation. I managed to successfully add a callback on a branch here (it doesn't do anything, but it builds) https://github.com/panda-re/panda/commit/ac6202594d3eae621528a1d248c71e5b3f072b69. This issue can be a placeholder for documentation, but we should write some docs and/or remove all the unnecessary manual effort with the context-based headers!