Migrate and re-organize lots of things about git, GitHub, and contributing

Created by: seancolsen

This PR does quite a bit of updating! I've tagged some people which I think might be interested in having a look.

Import content from:

• https://wiki.civicrm.org/confluence/display/CRMDOC/Contributing+to+CiviCRM+using+GitHub
  • Do a near-complete re-write of this content (that I think @JoeMurray wrote)
• https://wiki.civicrm.org/confluence/display/CRMDOC/Git+Commit+Messages+for+CiviCRM
  • Drop most of the content in this page that @totten wrote
• https://wiki.civicrm.org/confluence/display/CRM/Pull-Request+Process
• https://github.com/civicrm/civicrm-core/blob/master/.github/CONTRIBUTING.md
  • https://github.com/civicrm/civicrm-core/pull/10853 accompanies this PR by cleaning up CONTRIBUTING.md in core

Re-organize:

• https://docs.civicrm.org/dev/en/latest/core/contributing/
  • Split up content that @agh1 wrote. Put it into different areas of the Dev Guide and provide links to those areas within the "Contributing" page.
• https://docs.civicrm.org/dev/en/latest/core/develop/
  • Move useful content to new git page and delete this page

New pages on:

• Git, GitHub, and GitLab
• Issue tracking
• Jenkins
• Release process

New content on:

• Comparing our issue tracking systems
• Rebasing
• Managing multiple git remotes

Dropped content (which was in wiki pages) on:

• Super detailed steps for doing lots of git operations — I don't think we should maintain these sort of docs. I tried to offer some theoretical explanations of how to use git with CiviCRM, but I left a lot of the specific steps up to the reader to learn (and provided additional links for external resources)
• Screenshots of GitHub

Closes #61 (closed)

Reviews welcome. I will merge this in one week (on 2017-08-20) if there are no comments.