docs: CARS.md 2.1

[jyoung8607]

Design goals

• Preserve openpilot CARS.md location/behavior, supported cars only
• Start generating a new, separate CARS.md in opendbc
• opendbc's CARS.md will have less detail, but include many more cars
• don't break the website or the shop

Preview

• opendbc_repo/docs/CARS.md
• Documenting cars that aren't upstream: extra_cars.py
• Annotating upstream cars in dashcam: ford/values.py

To-do

• "atypical install" or something instead of "legacy", will include SecOC

[adeebshihadeh]

The classes don't feel quite right to me, but seems fine to start with. The infra and presentation look great.

[sshane]

Would a link to the under review PR be a good idea?

[jyoung8607]

I wrote the support link stuff with the idea we could override the link target, for any entry, to something outside the doc. A comma PR, a fork repo, something at docs.comma.ai, whatever. Or, we can make it a footnote style link, or whatever seems reasonable as we go.

Unrelated note, will hook up CI tests tomorrow AM to make sure new CARS.md is always up to date, same as openpilot.