Add docs theme toggle and update requirements.txt

mkdocs.yml

@@ -49,7 +49,14 @@ theme:
   favicon: img/favicon.ico
   logo: img/lens-logo-icon.svg
   palette:
-    scheme: preference
+    - scheme: default
+      toggle:
+        icon: material/toggle-switch
+        name: Switch to light mode
+    - scheme: slate
+      toggle:
+        icon: material/toggle-switch-off-outline
+        name: Switch to dark mode
   features:
     - navigation.instant