Restructure "Authoring Content" section

[smashwilson]

The organization of the "Authoring Content" section is not great right now. We have:

• "Opening blurb that's probably too technical"
• "Adding a new repository, the happy path"
• "Over-detailed explanation about content mapping that you probably don't need to know yet"
• "Tiny pull request builder section"
• "Way too much information about setup for non-happy paths"
• Then: "Links to specialized info about Jekyll/Sphinx."

I'd like to break it up and shuffle things around a little to be more useful. Here's what I'm thinking now:

• "Setting Up"
  • Summarize the prerequisites. A GitHub account, knowing where the control repository is, that kind of thing.
  • Existing "Adding a new repository" section.
• "Actually Writing"
  • Specialized Jekyll/Sphinx sections
  • Tips and command cheatsheet for git and GitHub workflow
  • Using the pull request builder
  • Troubleshooting: build failures, preparer failures, ...
• "Publishing"
  • "Where your content will live"
• "Special Cases"
  • "Custom Content Repositories" and other things that don't fit the happy path.