message: Add new auth providers to the doc index
diff: @@ -16,8 +16,10 @@ Backstage identity information in your app or plugins.
  Backstage comes with many common authentication providers in the core library:
  +- [Atlassian](atlassian/provider.md)
  - [Auth0](auth0/provider.md)
  - [Azure](microsoft/provider.md)
  +- [Bitbucket](bitbucket/provider.md)
  - [GitHub](github/provider.md)
  - [GitLab](gitlab/provider.md)
  - [Google](google/provider.md)
  