Tooling/Guide needs to be updated for comma's C3 support removal

[nelsonjchen]

comma will no longer support the C3 on their codebase. If the guide currently suggests or implements support using nightly or some comma codebase to install, we'll need to archive or fork or something for this.

I do expect sunnypilot to keep continued support but we might have an issue here if the guide suggests openpilot first.

[calvinpark]

Hopefully there's a good demarcation of what the last commit with C3 is

[nelsonjchen]

probably around this agnos update i think

[nelsonjchen]

https://github.com/optskug/openpilot/tree/nightly-dev-last-c3

https://github.com/optskug/openpilot/tree/nightly-dev-last-c3_neg_1

https://github.com/optskug/openpilot/tree/nightly-dev-last-c3_neg_2

have not tested on c3 though.

but we still might want to guide users to SP.