Fix developer documentation

This patch contains a few documentation fixes:
- Add missing gcc dependency to installation instructions
- Move CONTRIBUTING.rst and INSTALL.rst to docs folder
- Replace references to stackforge with openstack
- Replace GitHub references with git.openstack.org
- Correct invalid links
- Fix typos

Change-Id: I6922b84c6b4c6b1f517802a8b83bbdbd78a52c7d