refactor(theme-common): move useDocsPreferredVersion() to public api

[slorber]

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

useDocsPreferredVersion() is a useful hook to get the currently "selected" version (ie the one displayed in the navbar dropdown)

It is complicated to implement in userland and already used by some plugins (see also easyops-cn/docusaurus-search-local#205 (comment))

So let's move this to our public undocumented API: we maintain retro compatibility on all minor versions

Test Plan

CI + preview

Test links

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

Related issues/PRs

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	707da58
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/62c5ac9ba5c4d70008f2f8e2
😎 Deploy Preview	https://deploy-preview-7729--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 settings.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟢 90	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 82	🟢 100	🟢 100	🟢 100	🟢 90	Report

[Josh-Cena]

My take on this: if we make useDocsPreferredVersion public, we should do so for useLayoutDoc as well. I've found that to be more useful (and we do use it ourselves)

[github-actions]

Size Change: 0 B

Total Size: 800 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.6 kB
website/build/assets/css/styles.********.css	106 kB
website/build/assets/js/main.********.js	602 kB
website/build/index.html	38.9 kB

_{compressed-size-action}

[slorber]

We'll consider other internal apis to move to public in separate PRs