fix: Add aria-expanded for mobile navigation dropdown

[srapilly]

Pre-flight checklist

• 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

The mobile navigation dropdown does not convey the expanded state to assistive technologies such as a screen reader, it's also not focusable

https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-a-element

If the element has an href attribute: Interactive content.

It would be preferable to use an HTML button (a button should work with SPACE onKeyUp and ENTER onKeyDown) I think it could be done in another PR (styling changes would be needed)

Test Plan

Can be tested on Docusaurus website homepage with a zoom to trigger the mobile version:

• They should be keyboard focusable

• They should work with enter or space
• They should convey their expanded state

On the chrome accessibility tree view, we can see that:

• Versions has expanded: false
• Languages has expanded: true

Test links

Deploy preview: https://deploy-preview-9463--docusaurus-2.netlify.app/

Related issues/PRs

Quite similar to:

• Bug(navbar): languages and doc version drop downs are inaccessible via the keyboard and screen reader #8478
• A11Y improvements for DropdownNavbarItemDesktop #9439 (links for desktop dropdown)

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
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/docs/installation	🟠 54	🟢 98	🟢 100	🟢 100	🟠 89	Report
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/blog	🟠 64	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 53	🟢 97	🟢 100	🟢 100	🟠 89	Report
/blog/tags/release	🟠 84	🟢 100	🟢 100	🟠 80	🟠 89	Report
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