Update Basics

- Closing issues cross-repo now works
  See https://github.com/blog/1439-closing-issues-across-repositories

- Add missing keywords that can close an issue

- Update fork image

- Update forking and cloning instructions
  - Reduce to 3 steps
  - Explain better what's happening
  - Remove mentions of experimental since that branch no longer exists

- Remove Git Philosophy section
  Was out of place. I'll probably add some of those tips to Best Practices.

- Improve forking and cloning instructions
  - Fix a typo
  - Be more clear about what a fork is
  - Replace `git fetch upstream` with `git remote -v`.
    It's unlikely they need to fetch at that time, but making sure
    they set up the remote correctly is important.

- Rewrite Basic Workflow
  - Instructions are much more detailed and explained now
  - Command-line examples are given for appropriate steps
  - I threw in some good practices as well, so they learn them early

- Remove "Create Issue-Specific Branches"
  Section not needed anymore since it's covered in Basic Workflow

- Remove first footnote, no longer referenced

- Rewrite section on issue referencing

- Separate branch renaming steps and add explanation

- Add workflow diagram
  Note that I'm linking straight to the document on Google Drive,
  so whenever we update the diagram, we won't need to update the link.