fix(theme): improve a11y of DocSidebarItemCategory expand/collapsed button

[mxschmitt]

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

When using Narrator on Windows and using Tab to go over the items in the sidebar, it does not announce the expanded / collapsed state on link elements. This is because link elements should not be expandable, only buttons should be.

For expandable menu items with a preview page I moved the aria-expanded state to the actual button. For menu items without, I added role="button" to the element.

Test Plan

1. Use Narrator on Windows in the OS settings
2. Navigate to http://localhost:3000/docs/category/getting-started
3. Tab until you reach the sidebar

Expected: Narrator announces the collapsed / expanded state.
Actual: Does not

Test links

Deploy preview: https://deploy-preview-9944--docusaurus-2.netlify.app/

Related issues/PRs

Relates microsoft/playwright.dev#1265

This came out of a discussion with @scottaohara, who is in the W3C WAI-ARIA working group.

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⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 54	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 53	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 66	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 64	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 70	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report

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[slorber]

Thanks 👍