Refactor docs

[tavianator]

This is an alternative to #90, taking some but not all of its changes. Here is the preview.

I kept most of the (collapsed) README content as I think it's helpful on the front page. I do think some of those sections could be shorter, with a link to a separate document for all the details. Here's my rough TODO list:

• Move the "bfs adds some options that make common tasks easier" section to docs/Usage.md, just keeping a short summary in the README
• Split docs/CONTRIBUTING.md into docs/Building.md and docs/Hacking.md

[tavianator]

@ElectronicsArchiver @data-man What do you think about this approach?

[ElectronicsArchiver]

I can't say I prefer this..

[data-man]

I kept most of the (collapsed) README content as I think it's helpful on the front page.

Yes. And a small amount of browser redirects.