docs: refactor, move out of flake.nix

also filter source to limit rebuilds

dev/docs.nix

@@ -0,0 +1,27 @@
+{ config, pkgs, ... }:
+{
+  devShells.mkdocs = pkgs.mkShellNoCC {
+    inputsFrom = [
+      config.packages.docs
+    ];
+  };
+  packages = {
+    docs = pkgs.runCommand "docs"
+      {
+        buildInputs = [
+          pkgs.python3.pkgs.mkdocs-material
+        ];
+        files = pkgs.lib.fileset.toSource {
+          root = ../.;
+          fileset = pkgs.lib.fileset.unions [
+            ../docs
+            ../mkdocs.yml
+          ];
+        };
+      }
+      ''
+        cd $files
+        mkdocs build --strict --site-dir $out
+      '';
+  };
+}