Build docs only if they've changed

It's kind of wasteful when deploying the site to have to build texinfo and generate the manuals when they haven't changed. It would be nice if we didn't do that so that only when the manual sources have changed do we build and deploy them.

https://docs.gitlab.com/ee/ci/yaml/#onlyvariablesexceptvariables has some hints on how this might be done if the commit message contains some magic text.

https://docs.gitlab.com/ee/ci/yaml/#onlychangesexceptchanges describes how to do things only if files have changed (using glob patterns).

The html site can be built in all cases since they're relatively simple.