[docs] Improve Tree View selection doc

[flaviendelangle]

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My first goal was to add a single selection demo, which was really lacking. But I ended up doing a lot more changes on this page. I'll replicate the changes on the SimpleTreeView doc once approved :+1:

I took the DataGrid doc as an inspiration, but IMHO it is not great (no link to the multi select keyboard shortcuts for instance).

• [x] Add an example for the single selection
• [x] Link to the accessibility doc for both the single and multi selection sections
• [x] Link #12883
• [ ] Write the same doc on the SimpleTreeView

[mui-bot]

Deploy preview: https://deploy-preview-13105--material-ui-x.netlify.app/

Updated pages:

• docs/data/tree-view/accessibility/accessibility.md
• docs/data/tree-view/rich-tree-view/selection/selection.md
• docs/data/tree-view/simple-tree-view/selection/selection.md

Generated by :no_entry_sign: dangerJS against 2578e97c7c0a918a93cfcc087fcc23179232df86

[flaviendelangle]

Doc applied to SimpleTreeView and example added for parent-children relationship