Table of contents / outline feedback

[dmadisetti]

Just some quality of life suggestions for markdown headings.

• Currently only to h3 is supported (h4 - h6 missing from TOC panel #1617), maybe more would be considered if collapsable headings were enabled in the TOC?

• Another issue suggests collapsable blocks by heading: Feature request: Collapsible markdown headings #1085
  Super useful for large notebooks

• Colab has a "run section" button in their TOC, which is also nice

[akshayka]

@dmadisetti I also often use h4's; down to add support for that at least.

Collapsible headers would be awesome.