Revert font weight back to semibold for docs headings (#4496)

Author: veerapaananen

dspublisher/theme/global.css

@@ -30,7 +30,7 @@ html {
   --docs-font-size-h5: var(--text-size-md);
   --docs-font-size-h6: var(--text-size-sm);
 
-  --docs-font-weight-heading: normal;
+  --docs-font-weight-heading: var(--text-weight-semibold);
 
   --docs-line-height-s: var(--text-leading-sm);
   --docs-line-height-m: var(--text-leading-md);