docs: Update context menu codelab for v12 (re-try) (#2621)

* docs: Update context menu codelab for v12

* docs: Clean up mistakes

* docs: Run Prettier

* docs: Apply Prettier changes to doc

* docs: One more Prettier change to doc

* Replace 'click' with 'open' or 'select'

* Clarify how to restrict scope

* Clarify focusedNode

* Clean up wording

Author: rpbourret