[clang-tools-extra] [clang-doc] Make sidebar sections collapsible (PR #174531)

[Erick Velez via cfe-commits]

evelez7 wrote:

### Merge activity

* **Jan 6, 9:41 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.com/github/pr/llvm/llvm-project/174531).


https://github.com/llvm/llvm-project/pull/174531