[docs] List platform-level prerequisites in the contributing doc

[webknjaz]

https://coverage.readthedocs.io/en/7.10.6/contributing.html mentions git, make and tox as implied. People will probably figure out that tox needs to be installed somehow. But while looking into #2050, I noticed that tox does run but ends up with a bunch of test failures that don't make it obvious that gcc is missing, for example.

I propose at least mentioning mandatory stuff explicitly. In case of a clean Fedora 43 container, a command to get all the deps is dnf install -y gcc git make python3.13-freethreading python3.14-freethreading tox (I think make might be pulled in by gcc so that one may not be strictly necessary; plus the downstream tox RPM depends on a list of normal Python builds but the FT ones need to be retrieved explicitly).

If you don't want to maintain OS-specific commands, just listing git, make, tox as prerequisites plus optional Python runtimes in addition, could be a good middle ground.