Add table of contents icon to notebook toolbar

[claudiaregio]

We have still been seeing consistent feedback in the notebooks survey that table of contents for notebooks is hard to find (people are still asking for it in the survey as a new feature and mentioning it is frustrating not having one).

Since this has been very consistent feedback we should consider adding a table of contents icon to the notebook toolbar. (Icon request reference here: microsoft/vscode-codicons#80)