Update Git docs

First I only wanted to update and correct the steps listed on http://networkx.github.io/documentation/latest/developer/gitwash/following_latest.html, but then I saw that many links on http://networkx.github.io/documentation/latest/developer/gitwash/git_resources.html are also outdated. So I decided to clean that page and soft-redirect visitors to https://help.github.com/articles/what-are-other-good-resources-for-learning-git-and-github/ instead of listing outdated links.
subversion_ is a link in this case.