Fix Extension Docs (#2377)

Co-authored-by: Jim Ehrismann <40840436+jim-docker@users.noreply.github.com>

mkdocs.yml

@@ -35,6 +35,7 @@ nav:
       - Renderer Extension: extensions/guides/renderer-extension.md
       - Stores: extensions/guides/stores.md
       - Working with MobX: extensions/guides/working-with-mobx.md
+      - Protocol Handlers: extensions/guides/protocol-handlers.md
     - Testing and Publishing:
       - Testing Extensions: extensions/testing-and-publishing/testing.md
       - Publishing Extensions: extensions/testing-and-publishing/publishing.md