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.