nixos/documentation/modular-services: init

Render documentation for modular services.
https://nixos.org/manual/nixos/unstable/#modular-services

This is admittedly not a great solution, but it is a rather simple
solution that we can use until we develop a proper one.

Flaws:
- These are rendered in the NixOS documentation, but modular services
  are not meant to be exclusive to NixOS.
- They are rendered as NixOS options, but should be imported into
  service submodules.

Benefits:
- Simple
- search.nixos.org integration for free

nixos/modules/module-list.nix

@@ -139,6 +139,7 @@
   ./misc/assertions.nix
   ./misc/crashdump.nix
   ./misc/documentation.nix
+  ./misc/documentation/modular-services.nix
   ./misc/extra-arguments.nix
   ./misc/ids.nix
   ./misc/label.nix