Continuous integration

This PR converts the [Multiple development branch g.doc](https://docs.google.com/document/d/1LZkxRgzrfVcKttST2d1pNuViPXClb9ssD7Bvt1osRaQ/edit) into a proper Sphinx documentation page.

Objectives of this document:
- explain the dual development branch strategy
- describe the main Jenkins jobs used for testing the current & next major release
- introduce & describe some of the scc utility tools for managing branches & PRs