Fix duplicate Highlights anchor in 0.86 release post

[fabriziocucci]

Summary

The 0.86 release post showed two "Highlights" links in the sidebar table of contents, pointing to #highlights and #highlights-1.

The cause is two headings that both slug to highlights:

• the pre-truncate teaser ### Highlights
• the ## Highlights section heading after the truncate marker

Docusaurus deduped the second anchor to #highlights-1. In 0.86 the teaser only lists "Highlights", so the two entries sit adjacent in the sidebar and the duplication is obvious.

Fix

Promote the teaser to ## Highlights and drop the now-redundant ## Highlights section heading after the truncate marker. The highlight subsections (###) then nest under the single Highlights heading.

-### Highlights
+## Highlights

 - [A New Home for React Native](...)
 - [Edge-to-Edge on Android](...)
 - [React Native DevTools Improvements](...)

 {/* truncate */}

-## Highlights
-
 ### A New Home for React Native

Test plan

Verified against a local yarn build:

• only one id=highlights anchor is generated (no highlights-1)
• the sidebar shows a single Highlights with A New Home for React Native, Edge-to-Edge on Android, and React Native DevTools Improvements nested underneath it
• the teaser links (#a-new-home-for-react-native, #edge-to-edge-on-android, #react-native-devtools-improvements) all resolve