add list of backers

As advertised in our fund raiser, this PR enables the list of top backers.

The backers list is an html document. By default, Sphinx ignores non-.rst files, so I've modified some settings so that the `sphinxcontrib.rawfiles` extension is installed (hopefully). This extension will take care of copying over the new files. The list is wired up to a top-level topic.

I suppose we'll have to do a bit of trial-and-error with RtD to get this working.

@FichteFoll PTAL asap, as this should be a priority for us. Just quickly check that I haven't made any obvious mistakes.