docs: update broken links theme configuration page

[rbarbazz]

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

Updated broken links on this page.

Same question as in this other PR I just made, do we want to use links to the current master branch instead of permalinks?

Test Plan

N/A

Test links

Old link: https://docusaurus.io/docs/api/themes/configuration#theme
Deploy preview: https://deploy-preview-8976--docusaurus-2.netlify.app/docs/api/themes/configuration/#theme

Related issues/PRs

Related PR: #8975
Closes: #8968

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 77	🟢 97	🟢 92	🟢 100	🟠 89	Report
/docs/installation	🟠 81	🟢 100	🟢 92	🟢 100	🟠 89	Report