docs: Add lcov / codecov options for nox coverage and update docs

[alxhill]

The llvm-cov plugin needs to be installed as a cargo plugin (and at least on my system, had to run it manually once to respond to a user input prompt) before it works with nox.

nox -s coverage produces a coverage.json file and not lcov.info as of #3097, so also added an lcov option so nox -s coverage -- lcov returns the original format.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #3825 will not alter performance

_{Comparing alxhill:patch-1 (f29770f) with main (f04ad56)}

Summary

✅ 79 untouched benchmarks

[davidhewitt]

Yes, indeed the coverage job in noxfile.py has been hard-coded to prepare the coverage for codecov, mostly just for lack of investment. If you're interested, we could improve it to interpret session.posargs to be able to generate different formats.

[alxhill]

I'll give it a go later today - thanks for taking a look!