[$100 bounty] Speedup docs build in CI

[adeebshihadeh]

The build docs job in CI takes 3-5m each run. Last run from master: https://github.com/commaai/openpilot/actions/runs/9454981147/job/26043750673

This can probably be done by caching the build results, but non-cache speedups are even better. Build docs step must finish in <30s for the average run to qualify for the bounty.

[dpeachpeach]

I took a crack at this in #32766, is that kind of what you were looking for? It's a bit simplistic and doesn't speed anything up in case there is a cache miss so I can work to find other ways of speeding the actual build as well. Any feedback welcome!

[adeebshihadeh]

Closing since we're entirely redoing the docs