Rewrite documentation

[Osmose]

Rewrites a majority of our documentation. I think it's better, but do you think it's better? Who knows!

General source of inspiration here would be the documentation for Requests, with a bit of help from django-compressor and a few other libraries as well.

The setup doc is replaced by a Quickstart doc for getting people up and running much faster, and dumps most of the advanced use cases in the Customization doc. Troubleshooting also got a bit of a reorg to help people find their issues easily. Also includes some extra docs for upgrading from older versions (gleaned from looking at diffs of the setup docs between tags).

I'm pretty happy with the result, but I know that a good thrashing in review will help iron out the wrinkles.

Also, a few other nice things:

• Removes context_processors.py. No idea why that wasn't deleted when we stopped using it.
• Sets us up to use the ReadTheDocs custom theme when building locally, so we can get a better idea of what the docs will look like.
• Adds CONTRIBUTING.rst to the root of the repo so that it shows up in the Github pull request page, and includes these in the generate docs. Will be nice to be able to point people to standards instead of just notifying them individually of what they did wrong.

[Osmose]

For review, I respectfully summon @jezdez for his documentation prowess (if you have time, of course :D) and @willkg and @peterbe for their django-browserid prowess (again, feel free to push this off if you're too busy, surely one of three people will be able to look!). <3

[willkg]

I finished my docs pass. I did mostly a "stream-of-consciousness review" where I wrote down things as I thought of them.

I think there are two missing things:

1. It needs a "Requirements" section that's easy to find. I'd put it on the front page or give it its own chapter named "Requirements" that's listed pretty early in the TOC. It'd list requirements for Django, requests, jquery and whatever else is required to use django-browserid
2. There should be a Quickstart for Jinja2 templates. Mozilla sites all use Jinja2. Given that, it seems a common use case for us to support. Having the Jinja2 bits squirreled away in the "Customization" section will make it hard to find for Moz folks.

It's a big overhaul--looks good so far!

[Osmose]

Updated based on feedback. Ignored one or two things and commented on why they were ignored.

[willkg]

I think this looks good for me. r+