Update docs for github migration

[mperrin]

We need to:

• [x] Update installation documentation for source code install.
• [ ] Update the pipeline_dev build script to point to github instead of svn.
• [ ] Update developer docs: developers/intro.rst, developers\releases_and_versioning.rst, developers\new_primitives.rst
• [ ] Update "New GPI Team Members Start Here" google doc
• [ ] Update pipeline version number display to show git commit hash instead of svn id.

[mperrin]

I will try to do most of this today.