docs: add aria tree examples and disable text selection

[ok7sai]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

• Add disabled-focusable examples and single-select-follow-focus examples.
• Disable text selection on the tree examples.
• Add some default selections to examples.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[bencodezen]

Love it. Thanks for the PR @ok7sai!

[thePunderWoman]

This PR was merged into the repository. The changes were merged into the following branches:

• main: 24814ce
• 21.0.x: 4087570

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