fix(theme-classic): remove redundant sidebar label titles

[cyphercodes]

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 Accessibility: Reduntant title text in sidebar #11945) and the maintainers have approved on my working plan. (Not a new API/substantial change.)

Motivation

Fixes #11945.

WAVE reports redundant title text for docs sidebar labels because the label text is rendered both as visible text and as an identical title attribute. This removes the redundant title attributes from docs sidebar link and category label spans while preserving the existing visible label markup and styling.

AI-assisted: I used AI assistance while preparing this PR and verified the resulting changes locally.

Test Plan

• YARN_IGNORE_ENGINES=1 yarn test packages/docusaurus-theme-classic/src/theme/DocSidebarItem/__tests__/index.test.tsx --runInBand
• YARN_IGNORE_ENGINES=1 yarn eslint --cache --report-unused-disable-directives packages/docusaurus-theme-classic/src/theme/DocSidebarItem/Link/index.tsx packages/docusaurus-theme-classic/src/theme/DocSidebarItem/Category/index.tsx packages/docusaurus-theme-classic/src/theme/DocSidebarItem/__tests__/index.test.tsx
• yarn prettier --check packages/docusaurus-theme-classic/src/theme/DocSidebarItem/Link/index.tsx packages/docusaurus-theme-classic/src/theme/DocSidebarItem/Category/index.tsx packages/docusaurus-theme-classic/src/theme/DocSidebarItem/__tests__/index.test.tsx

I also attempted YARN_IGNORE_ENGINES=1 yarn workspace @docusaurus/theme-classic build after installing dependencies with scripts disabled, but it could not complete because local workspace packages such as @docusaurus/plugin-content-blog/client and @docusaurus/plugin-content-docs/client had not been built in this checkout.

Test links

Deploy preview: https://deploy-preview-11966--docusaurus-2.netlify.app/

Related issues/PRs

• Fixes Accessibility: Reduntant title text in sidebar #11945

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