- Organise docs by: Overview - Code - Docs - Community
- Split out some content into separate files to make it more
  obvious where to find the content
- Simplify new contributor docs
- Add docs_layout to mirror the dir_layout file
- Make docs style guide separate
- Move introduction to explanation and include front page
  link to help direct people to it

Co-authored-by: James Falcon <james.falcon@canonical.com>