Adds a NixOS module which allows using mandoc as the main manual
viewer. It can be used as a drop-in replacement for documentation.man
which relies on GNU's man-db and provides more or less the same
features.
The generateCaches option requires a different implementation for
mandoc, so it is hard to share code between the two modules -- hence it
has been implemented separately. Using both at the same time makes
little sense and wouldn't quite work, so there's an assertion to
prevent it.
To make makewhatis(8) index manual pages which are symlinks to the nix
store, we need to set READ_ALLOWED_PATH to include
`builtins.storeDir`. For background and discussion see:
https://inbox.vuxu.org/mandoc-tech/c9932669-e9d4-1454-8708-7c8e36967e8e@systemli.org/T/
