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

[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
  • (NFC) Improve docs in CONTRIBUTING.md civicrm-core#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

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

[seancolsen]

I'm closing this PR because I (accidentally) merged these commits when I merged #273. (I think I must have based the branch for #273 on the branch for this PR by mistake. Ooops!!)

I still welcome reviews on these changes and am happy to make further edits in the published guide. Also I'll work on being more careful when creating new branches, taking care to always explicitly base them on master.