Document cpython-source-deps repo

[Mariatta]

I learned of the purpose of https://github.com/python/cpython-source-deps repo while working on https://github.com/python/cpython/pull/4246 :)

It should be documented somewhere in the devguide, not sure where.

Maybe under "Additional Repositories" in https://devguide.python.org/communication/ ?

[ncoghlan]

The section on getting set up with the build dependencies would be the right place: https://devguide.python.org/setup/#build-dependencies

Since these are mainly useful for Windows (where we can't use the system package manager or homebrew to get them), I'd suggest the Windows-specific subsection: https://devguide.python.org/setup/#windows