CONTRIBUTING.md: Replace AstroPy links with GitHub's Flow guide

GitHub just deployed some guides [1], so we can reverence them (with their
pretty graphics and accessible UI) instead of the AstroPy docs.

GitHub's guide is pretty, but it doesn't go into lots of technical detail
(e.g. how do I make a commit or open a pull request?), so we still need a link
to a Git tutorial.  I've replaced the old AstroPy link with one to our own
novice lesson, since we might as well eat our own dog food.

[1]: https://github.com/blog/1769-announcing-guides