Show "Platforms" eyebrow on landing page (#238)

This module belongs to the "Platforms" nav group in the combined
Swift docs site, but the landing page showed no eyebrow above the
title.

Author: heckj

Sources/EmbeddedSwift/EmbeddedSwift.docc/EmbeddedSwift.md

@@ -16,6 +16,7 @@ Boards with active community support include the Raspberry Pi Pico, various STM3
 
 @Metadata {
     @TechnologyRoot
+    @TitleHeading("Platforms")
 }
 
 ## Topics