Reorganize the existing documentation in the Intern repo. The goal of this issue isn't to substantially rewrite or add documentation (not all of the topics in the example below exist yet), but to reorganize and prune the existing docs in preparation for future updates.

See #1088 for discussion.

The overall structure should end up like:

• Getting Started
  • Installing
  • Unit testing (the basics of writing unit tests for Node and browsers)
  • WebDriver testing (the basics of Intern's WebDriver API)
  • Running Intern (cli and browser client, environments, watch mode)
• Guides
  • Command line options
  • Configuration
  • Code coverage
  • Using with Babel
  • Using with TypeScript
  • Using with ES modules
  • Mocking
  • ...
• API

Documentation will still be kept in the docs/ directory in the repo. The overall table of contents should remain in the README; currently it's in a "More Information" section, which should be renamed to "Documentation".

Within the docs directory, individual topics should generally be one per file. If a group of related topics are short, or it makes sense for them to be read continuously, they can go in one file. The "getting started" topics should probably be in the doc root, the guides can go in a "guides" subdirectory, and API docs can go in an "api" subdirectory.