feat: nix: add a new mise task and nix flake output to generate NixOS docs

Adds a new Nix flake output `headplane-nixos-docs` which generates a
Markdown page with all NixOS `services.headplane.settings.*` options.

Replaces existing handwritten options documentation.

nix/docs.nix

@@ -0,0 +1,24 @@
+{
+  lib,
+  nixosOptionsDoc,
+  runCommand,
+  ...
+}: let
+  eval = lib.evalModules {
+    modules = [./options.nix];
+  };
+  transformOptions = opt:
+    if (lib.hasPrefix "_" opt.name)
+    then
+      opt
+      // {
+        internal = true;
+        visible = false;
+      }
+    else opt;
+  optionsDoc = nixosOptionsDoc {
+    inherit (eval) options;
+    inherit transformOptions;
+  };
+in
+  runCommand "headplane-nixos-docs.md" {} "cat ${optionsDoc.optionsCommonMark} > $out"