With upcoming changes to the dovecot home and maildirectories we need to
introduce a way to nudge users to inform themselves about manual
migration steps they might need to carry out.
The idea here is to allow us to safely make breaking changes and notify
the user of required migration steps at eval time, so they can make the
necessary changes in time.
To generate the list of options, we need to generate and commit a rst
file to make all files available for ReadTheDoc.
An Hydra test ensures this generated file is up-to-date. If it is not
up-to-date, the error message explains the user how to generate it:
the user just needs to run `nix-shell --run generate-rst-options`.
