enable GitHub Pages for model-transparency

We are hosting library documentation via GitHub Pages, which requires
the feature enabled. For details see
sigstore/model-transparency#209.

Signed-off-by: Spencer Schrock <sschrock@google.com>

github-sync/github-data/sigstore/repositories.yaml

@@ -962,6 +962,9 @@ repositories:
       - name: model-transparency
         id: 10329477
         permission: push
+    pages:
+      branch: gh-pages
+      path: /
     branchesProtection:
       - pattern: main
         enforceAdmins: true