✨ feat: allow pinned posts with pagination

[welpo]

Summary

Adds support for pinned posts when using pagination (paginate_by). Previously, pinned posts only worked when using section_path.

Also restricts pinned posts to appear only on root pages (e.g., /, /es/, /fr/).

Related issue

Should've probably been added in #424.

Changes

• Handle pinned posts with pagination
  • Show pinned posts only on page 1
• Restricted pinned posts to root pages only (language-aware)
• Added clear documentation about pinned posts behavior with pagination

Type of change

• Bug fix (fixes an issue without altering functionality)
• New feature (adds non-breaking functionality)
• Breaking change (alters existing functionality)
• UI/UX improvement (enhances user interface without altering functionality)
• Refactor (improves code quality without altering functionality)
• Documentation update
• Other (please describe below)

---

Checklist

• I have verified the accessibility of my changes
• I have tested all possible scenarios for this change
• I have updated theme.toml with a sane default for the feature
• I have made corresponding changes to the documentation:
  • Updated config.toml comments
  • Updated theme.toml comments
  • Updated "Mastering tabi" post in English
  • (Optional) Updated "Mastering tabi" post in Spanish
  • (Optional) Updated "Mastering tabi" post in Catalan

[netlify]

✅ Deploy Preview for tabi-demo ready!

Name	Link
🔨 Latest commit	1956d4b
🔍 Latest deploy log	https://app.netlify.com/sites/tabi-demo/deploys/6735349c7429320008a6386f
😎 Deploy Preview	https://deploy-preview-428--tabi-demo.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.