[Accessibility Docs] List tree

Using [this template](https://docs.google.com/document/d/1IwFMfJrTfJPXDcVJfLWXYkw-pjgWhBWo4GX8X098Q64/edit#), create accessibility docs for [List tree](https://vanillframework.io/docs/patterns/list-tree)

Start with the linked accessibility documentation template and the List tree pattern page at vanilla-framework.io/docs/patterns/list-tree. Create the accessibility documentation for that pattern using the template; done means the List tree docs are completed and available in the project documentation.

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