Replace strict with warnonly and make makedocs builds fail by default

[mortenpi]

Close #2051.

[mortenpi]

This should be good to go now.

As for linkcheck: I've been going back and forth in my head about this, but my feeling right now is that we should keep that as false by default, because it's a bit flaky and also environment dependent. The checks that are on by default should be consistent. So the user should consciously opt-in to the online linkchecks.

One thing to note, though, is that if you set linkcheck = true, then linkcheck failures now do fail the build (unless you set warnonly = :linkcheck).