Better URLs for pip as example project.

Old URLs made more sense in original text that used astropy as example.
Small rephrasing.
Merge pull request #566 from toffer/redirect-docs

More tweaking of the redirect documentation.
Added bitbucket.org urls, since they're the canonical ones anyway
Merge pull request #567 from DanielOaks/bitbucket-fix

Fix a bug with 'Edit on Bitbucket' with .org urls
Just use .org for bitbucket
Add bit about rtfd.org