docs: document more experiments with overlays in docs

doc/overlays/overlay2/src/my/twitter/__init__.py

@@ -0,0 +1,13 @@
+print(f'[overlay2] {__name__} hello')
+
+from pkgutil import extend_path
+__path__ = extend_path(__path__, __name__)
+
+def hack_gdpr_module() -> None:
+    from . import gdpr
+    tweets_orig = gdpr.tweets
+    def tweets_patched():
+        return [t.replace('gdpr', 'GDPR') for t in tweets_orig()]
+    gdpr.tweets = tweets_patched
+
+hack_gdpr_module()