Generate user documentation.

With neovim/neovim#1011, HTML user documentation can be generated. If the [build](https://travis-ci.org/fwalch/bot-ci/builds/31241189) passes, the user docu should show up at https://fwalch.github.io/doc/user.

I used a similar approach to the clang report to wrap the docu in Neovim's layout. Unfortunately, the contents of eval.html (9000 lines or so?) were too much for envsubst, so I had to modify `generate_report` to echo the body directly.