docs(website): deploy generated docs on gh-pages branch

[0x-r4bbit]

With the release of clog's new website, we should also make the documentation of the project accessible. AFAIK, the documentation is already generated automatically (which is awesome). We now need to deploy it to gh-pages branch, so it can be served by GitHub.

[kbknapp]

This can be done via travis-cargo which is what I use to accomplish this with clap. @cburgdorf or @PascalPrecht will have to set it up though because it requires giving Travis-CI an OAuth token and write access to the clog repo.