Fix duplicate Highlights anchor in 0.86 release post

The pre-truncate teaser "### Highlights" and the "## Highlights"
section heading both slug to "highlights", so Docusaurus generated two
table-of-contents entries (#highlights and #highlights-1).

Make the teaser bold text instead of a heading. The "## Highlights"
section now appears once in the sidebar with its three subsections nested
underneath it, and the teaser preview links still resolve.

Author: fabriziocucci

website/blog/2026-06-11-react-native-0.86.mdx

@@ -13,7 +13,7 @@ This release includes comprehensive edge-to-edge support on Android 15+ and impr
 
 Following 0.83, this is the second React Native release with no user-facing breaking changes, reflecting our continued commitment to making upgrades more predictable and seamless.
 
-### Highlights
+**Highlights**
 
 - [A New Home for React Native](/blog/2026/06/11/react-native-0.86#a-new-home-for-react-native)
 - [Edge-to-Edge on Android](/blog/2026/06/11/react-native-0.86#edge-to-edge-on-android)