[meta] Handle non-bikeshed specs in build-specs.yml

Fixes w3c#8824

Author: Loirooriol

.github/workflows/build-specs.yml

@@ -41,6 +41,11 @@ jobs:
       - name: Build specs
         run: |
           set -e
+          # Handle non-bikeshed specs.
+          for file in ./**/Overview.html; do
+            cp "$file" "$(dirname "$file")/index.html"
+          done
+          # Handle bikeshed specs.
           for file in ./**/Overview.bs; do
             # We use `date` to build a YYYY-MM-DD date rather than using git's
             # `--format=%as` because we want the date not to depend on the