Fix title_meta handling in doc front matter

[MisterFISHUP]

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 title_meta field, as documented in the docs, should allow overriding frontMatter.title for SEO purposes. However, it doesn't work for doc pages, while it does work correctly for blog posts.

I tried to fix the issue in docs by following how it works for blog pages.

Related issues/PRs

#10586, #10878

[netlify]

✅ [V2]

Built without sensitive environment variables

Name	Link
🔨 Latest commit	c6c3cc6
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/679981a296fe680008b99ffa
😎 Deploy Preview	https://deploy-preview-10879--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	Report
/	🟠 71	🟢 98	🟢 96	🟢 100	Report
/docs/installation	🟠 51	🟢 97	🟢 100	🟢 100	Report
/docs/category/getting-started	🟠 73	🟢 100	🟢 100	🟠 86	Report
/blog	🟠 63	🟢 96	🟢 100	🟠 86	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 62	🟢 92	🟢 100	🟢 100	Report
/blog/tags/release	🟠 64	🟢 96	🟢 100	🟠 86	Report
/blog/tags	🟠 74	🟢 100	🟢 100	🟠 86	Report

[slorber]

This feature has only be added to the blog plugin on purpose, as a blog-only temporary workaroud

For docs, it's not really needed: frontMatter.title is already enough for that purpose

See my comment here: #10586 (comment)

If you still think this feature needs to be added to the docs plugin, I'd really like to know why. Please give a concrete example where you couldn't find any alternative, and we'll consider it.

---

However you caught an interesting mistake: we implemented this for the blog plugin, but documented it in the docs plugin API ref 😅 The docs need to be fixed.

#10586