Enable GitHub Pages for model-transparency

[spencerschrock]

Summary

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

Release Note

NONE

Documentation

NONE

[github-actions]

🍹 preview on sigstore-github-sync/sigstore/github-prod

Pulumi report

  Previewing update (sigstore/github-prod)

View Live: https://app.pulumi.com/sigstore/sigstore-github-sync/github-prod/previews/43f97542-e858-4b85-9b29-87d73b29a15b

@ Previewing update.....
pulumi:pulumi:Stack: (same)
  [urn=urn:pulumi:github-prod::sigstore-github-sync::pulumi:pulumi:Stack::sigstore-github-sync-github-prod]
  ~ github:index/repository:Repository: (update) 🔒
      [id=model-transparency]
      [urn=urn:pulumi:github-prod::sigstore-github-sync::github:index/repository:Repository::model-transparency]
      [provider=urn:pulumi:github-prod::sigstore-github-sync::pulumi:providers:github::default_6_2_3::dfb01c0f-bb2a-45c2-9d92-e6152b5cee06]
    + pages: {
        + buildType : "legacy"
        + source    : {
            + branch    : "gh-pages"
            + path      : "/"
          }
      }
    + pages: {
        + buildType : "legacy"
        + source    : {
            + branch    : "gh-pages"
            + path      : "/"
          }
      }
@ Previewing update....
Resources:
  ~ 1 to update
  591 unchanged
  

[spencerschrock]

Looking into the failure, I copied what sigstore-python does (which doesn't have a gh-pages branch).

[haydentherapper]

422 The gh-pages branch must exist before GitHub Pages can be built.