Only build the documentation on protected branches

The nightlies we depend on can fail, but this isn't solvable by MR authors so they shouldn't be faced with a failing pipeline. Instead, only rely on the nightlies on protected branches, which are the responsibility of Moore maintainers.

Closes #115 (closed).

(A safer choice of slot would help, say using lhcb-master rather than lhcb-head, but master will break at some point, and then MR authors will still be confused as to why their pipelines are failing. I would love to build the docs in every MR, but it seems too much of a pain at the moment. /cc @clemenci in case you have any ideas!)