chore: move docgen comment to markdown file

docs/src/docGen.md

@@ -0,0 +1,21 @@
+# Documentation
+Docs are generated from comments in `.nix` files.
+This results in kind of weird config format, but
+makes it easier to track configuration changes
+and add/remove docs as required.
+
+## How docs are generated
+
+`nixdoc` utility enables us to generate documentation
+from comments in `.nix` file, like `doxygen` and `rustdoc` do.
+
+`nixdoc` requires this flags:
+- category (derived from file's path)
+- description (derived from file's first line if it's commented)
+
+most hard part is to generate SUMMARY.md.
+generator uses orderKey of type `{num}|{mdPath}` and derives indentation from path's depth,
+so chapters have proper order.
+
+run `nix build .#docBuild` to build documentation.
+run `nix run .#docServe` to serve documentation site