Automate BridgeDB documentation builds

The developer documentation for BridgeDB needs a place to live. Currently, Sphinx builds can be manually triggered to produce HTML documentation (as well as other types). I have been manually scp'ing it to https://para.noid.cat/bridgedb, mostly because I had no idea where to put it, but wanted to put it somewhere.

So...

1. We should probably ask TPO's sysadmins to create something like https://docs.torproject.org/bridgedb, or wherever people want to use for keeping common developer documentation.

2. I think there was some sort of funding for this. I've just been doing this because I needed to understand how BridgeDB works and it was entirely undocumented when I started working on it. If someone knows who that funder is, please comment/tag this ticket as appropriate.

3. Documentation builds should probably be triggered automatically when a new signed tag is pushed to git.torproject.org/bridgedb.git.