fix(man): switch to mandoc backend so nix man pages are discoverable on darwin

home-manager's programs.man defaults install man-db, which doesn't index the
profile for macOS BSD man(1). Disabling man-db and enabling mandoc symlinks
profile pages into ~/.local/share/mandoc/man and prepends it to MANPATH, so
`man <pkg-from-nix>` resolves the page.

Author: charliie-dev

modules/apps/man.nix

@@ -1,5 +1,7 @@
 {
   man = {
     enable = true;
+    man-db.enable = false;
+    mandoc.enable = true;
   };
 }