Allow control of which sections are automatically expanded (expand_sections)

[firasm]

Is your feature request related to a problem? Please describe.

I saw in PR1073 that expand_sections was deprecated.

I really liked the ability to expand certain sections of my jupyterbook by default on my course websites. I set the current week’s material to automatically expand so the students know what we’re doing this week.

Describe the solution you'd like

Setting the expand_sections to true for each part of the TOC was a good solution.

Describe alternatives you've considered

The navbar depth seems to be a global setting:

html:
  show_navbar_depth: 1

Additional Context

From @choldgraf in #1134 :

wanna open up a new issue to discuss adding expand_sections back in? It would require new development since the machinery for the sidebar has changed, so we should track it as a new feature.

[firasm]

If someone has any suggestions or pointers on how I could potentially implement this myself, I would like to give it a shot since it probably only applies to me 😂