fix(theme): mobile drawer sidebar should use native modal dialog and backdrop

[hidde]

This uses the <dialog> element with showModal() method, so that we can leverage the browser's built-in ways to make this dialog modal, including keyboard handling (Esc), backdrop and focus trap.

Fixes the issue that it was possible to tab outside the dialog while it was open.

Rmoves the backdrop component, as the <dialog> element comes with a backdrop built-in, that is styled via CSS instead.

fixes https://github.com/facebook/docusaurus/issues/9457

Pre-flight checklist

• [x] I have read the Contributing Guidelines on pull requests.
• [ ] If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• [ ] If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

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Deploy preview: https://deploy-preview-10706--docusaurus-2.netlify.app/

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[hidde]

@slorber : ~I'm not sure how to make that last autolintfix check run, please advise.~ Linted locally so autolintfix no longer needed

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Build	Status	Details	Updated (UTC)
default (Inspect)	✅ No changes detected	-	Feb 27, 2025, 4:05 PM