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