up to now, those were managed outside of this repo, which as
unsurprisingly a real hassle to deal with if one wanted to prevent URLs
from breaking when moving pages around. this change removes a large part
of the friction involved in moving content in the Nix manual.
possible next steps for further automation:
- check for content that moved and warn if it's not reachable from
links that were valid prior to a change
- create redirect rules automatically based on this information
