Since the help sources extend the Help menu, putting them under Extensions makes some sense.  But I don't know how many people would think to look at the tab.  But something to keep in mind.  We can also ask for user opinions on idle-dev list.

Notebooks can put tabs on any side.  I found one side example.
https://stackoverflow.com/questions/46909169/python-tkinter-side-notebook-tabs
This would allow more and longer tab names, though I would prefer position='w'or 'wn' (west north) instead of 'ws'.  We should try this for the existing extensions pane. 

When done with General, there are issues about redoing Highlights and Keys pages, with one person's mockups.  One problem with making changes has been divergent opinions.