Add the "overview" tab

[SeanMcCarren]

Still needs improvement

Highlights:

• overview tab on the right (next to other assistance buttons)
• need to click the title "Overview" to update the overview (looking for improvements). This finds <h[1-6]> tags in notebook and stores them, along with indices
• formatting is also not great but I like that we could fit it in the side-window format.
• can press on the table indices to navigate to them, but in notebooks you automatically edit the text which may be weird

[SeanMcCarren]

I wanted to give this another shot... I think it is improved because the previous 'set-focus' had the disadvantage of opening a CodeMirror, regardless of whether a user should be able to open a CodeMirror (shouldn't be possible outside of input blocks in wpn for example). I changed the formatting slightly, but this might still be improved. Thoughts?