New homepage, begin refactor

- New text for front page
- New proposed docs structure
- Deleted a lot of stuff
- Moved other stuff around
- theme_overrides.css isn't working…?
Add 'get a dashboard' page and roadmap stub
Re-add integrating section
Re-add TOC
Remove user-manual folder for the time being
Cleanup before merge
Merge pull request #37 from alphagov/henrys-refactor-2

Docs clean-up