Collapsible sections in the table-of-contents

It is hard to navigate the [C-S API](https://spec.matrix.org/v1.10/client-server-api/) index. What about collapsing by default any section or subsection with more than a certain number of children? It could be expanded with a click on an arrow, at the right of the subsection header. For example

```
API Standards ▼

Standard error response ▼

Common error codes
Other error codes
Rate limiting

Transaction identifiers
```

Start by inspecting the C-S API table of contents at the linked specification page and identify where its sections are generated. Define the child-count threshold and confirm that qualifying sections are collapsed by default, expandable with an arrow, and still allow the listed subsections to be reached.

Written by the indexing model from the issue text.