Update gitdevs.rst for Git/GitHub

I don't' know if it should just be deleted or reworked to document the preferred workflow (although that should be covered by other pages).

The only reason I can think of keeping this around is that a very select number of repos will probably stick around on hg for a little while (e.g. the bugs.python.org repo). But since the doc is already written then the opening paragraph should be tweaked to not suggest that CPython uses Mercurial and then leave it be.