refactor: extract useSkipToContent()

[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

move useSkipToContent() technical code to theme-common

Test Plan

no change

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	a7254ec
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/626c17a8f87d970009b3af28
😎 Deploy Preview	https://deploy-preview-7269--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 changes in this PR:

Category	Score
🟠 Performance	65
🟢 Accessibility	100
🟠 Best practices	83
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-7269--docusaurus-2.netlify.app/

[github-actions]

Size Change: +129 B (0%)

Total Size: 806 kB

Filename	Size	Change
website/build/assets/js/main.********.js	609 kB	+129 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	50.7 kB
website/build/assets/css/styles.********.css	107 kB
website/build/index.html	38.8 kB

_{compressed-size-action}

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	9c63c4f
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/626c1829e8420800089a123e
😎 Deploy Preview	https://deploy-preview-7269--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.