Block merging of EN docs if Weblate PR open

[ignotus666]

If a PR with changes to (translatable) EN docs is merged while there is a Weblate PR open, it leads to conflicts that make Weblate lock up.

One way to address this might be to automatically add a "Check for Weblate PR" label to PRs that involve translatable EN docs. Merging would be blocked unless this label is removed. Of course this could be bypassed by removing the label and merging regardless of whether there's a Weblate PR, but I think that's improbable - we just need a reminder.

[pljones]

Yeah, good idea.