chore: add initial nixmkdocs setup and improve documentation

.nix/repo/docs.nix

@@ -0,0 +1,68 @@
+{inputs, ...}: let
+  inherit (inputs) pkgs devshell doclib;
+
+  roots = [
+    {
+      url = "https://gitlab.com/rensa-nix/devshell/-/blob/main/lib";
+      path = "${inputs.self}/lib";
+    }
+  ];
+  optionsDoc = doclib.mkOptionDocs {
+    module = devshell.modules;
+    inherit roots;
+  };
+  optionsDocs = pkgs.runCommand "options-docs" {} ''
+    mkdir -p $out
+    ln -s ${optionsDoc} $out/options.md
+  '';
+in
+  (doclib.mkDocs {
+    docs."default" = {
+      base = "${inputs.self}";
+      path = "${inputs.self}/docs";
+      material = {
+        enable = true;
+        colors = {
+          primary = "aqua";
+          accent = "blue";
+        };
+        umami = {
+          enable = false;
+          src = "https://analytics.tf/umami";
+          siteId = "";
+          domains = ["devshell.rensa.projects.tf"];
+        };
+      };
+      macros = {
+        enable = true;
+        includeDir = toString optionsDocs;
+      };
+      config = {
+        site_name = "Devshell";
+        repo_name = "rensa-nix/devshell";
+        repo_url = "https://gitlab.com/rensa-nix/devshell";
+        theme = {
+          logo = "images/logo.png";
+          icon.repo = "simple/gitlab";
+          favicon = "images/favicon.png";
+        };
+        nav = [
+          {"Introduction" = "index.md";}
+          {"Options" = "options.md";}
+        ];
+        markdown_extensions = [
+          {
+            "pymdownx.highlight".pygments_lang_class = true;
+          }
+          "pymdownx.inlinehilite"
+          "pymdownx.snippets"
+          "pymdownx.superfences"
+          "pymdownx.escapeall"
+          "fenced_code"
+        ];
+      };
+    };
+  }).packages
+  // {
+    inherit optionsDocs;
+  }