Provide nesting in TOC for sections

[stevejpurves]

Context

Given a valid _toc.yml of:

format: jb-book
root: index
parts:
  - caption: Tutorials
    chapters:
      - file: tutorial1
      - file: tutorial2
        sections:
          - file: tutorial2.1
          - file: tutorial2.2
      - file: tutorial3

mystmd produces a TOC data structure that reflects grouping and nesting via the level field:

"pages": [
{
"title": "Tutorials",
"level": 1
},
{
"slug": "tutorial1",
"title": "TUTORIAL 1",
...,
"level": 2
},
{
"slug": "tutorial2",
"title": "TUTORIAL 2",
...,
"level": 2
},
{
"slug": "tutorial2-1",
"title": "TUTORIAL 2.1",
...,
"level": 3
},
{
"slug": "tutorial2-2",
"title": "TUTORIAL 2.2",
...,
"level": 3
},
{
"slug": "tutorial3",
"title": "TUTORIAL 3",
...,
"level": 2
}
]

But the myst theme TOC does not show this nesting.

Proposal

1. nest the TOC items in line with the level reported in the data structure
2. enable expand/collapse controls for items level > 2

Tasks and updates

No response

[rowanc1]

An initial take was put into #254. I think there are still some things to do, but that should fix this specific issue.