next.js

a03b8eee - docs: link Activity guide from Cache Components migration guide (#92571)

docs: link Activity guide from Cache Components migration guide (#92571) ## Summary - Add a "UI state preservation" section to the Cache Components migration guide explaining that pages are now preserved via React's `<Activity>` instead of unmounting on navigation - Add `related` links pointing to the preserving UI state guide and `cacheComponents` config reference - Cross-link the migration guide from the preserving UI state guide for bidirectional discovery The migration guide previously only covered route segment config replacements (`dynamic`, `revalidate`, `fetchCache`, `runtime`) but didn't mention the fundamental navigation behavior change. Users migrating to Cache Components need to know that cleanup logic, analytics events, and state resets may behave differently. --------- Co-authored-by: Joseph Chamochumbi <joseph.chamochumbi@vercel.com>