Publish docs from CI

[kmicklas]

We have a Nix derivation to build the docs. Pushing to master (or maybe a release branch) should upload to GitHub pages.

A prerequisite for this is getting all the examples to compile again.

[tathougies]

Which examples don't work? I can go through them.

[kmicklas]

I fixed the failures in #613 so all that's left is to automate publishing.