Documentation: Restructured toplevel documentation

This is intended to make things easier to find for the
various types of people searching for stuff.

  o The installation instructions remain on the main page.

  o Three main separate pages have been created
    - Using BuildStream
    - Authoring BuildStream projects
    - Core API reference, for plugin authors

  o The "Authoring projects" section swallows the
    previous plugin index; so one can find the plugin one
    is looking for on the same page as the rest of the format
    docs

  o The plugin authoring section has been swallowed by the
    core API reference section, with a note that this is useful
    especially for plugin authors.