docs: OIDC support for other CI providers

[weikanglim]

With #5397, azd supports OIDC for other CI providers, i.e. GitLab. This issue tracks any doc updates related to the feature.

[weikanglim]

Some suggestions I was thinking about:

• Mention this in the release blog post. It could be worthy to put in a small e2e sample (like the one for GitLab below)
• Possible mention in https://learn.microsoft.com/en-us/azure/developer/azure-developer-cli/pipeline-advanced-features for:
  • The flag to enable the feature: --federated-credential-provider oidc
  • The variables: AZURE_OIDC_TOKEN and AZURE_OIDC_REQUEST_TOKEN/AZURE_OIDC_REQUEST_URL

A minimal GitLab example would look something like:

variables:
  AZURE_CLIENT_ID: "<client-id>"
  AZURE_TENANT_ID: "<tenant-id>"

auth:
  id_tokens:
    AZURE_OIDC_TOKEN:
      aud: api://AzureADTokenExchange
  script:
    - azd auth login --client-id $AZURE_CLIENT_ID --tenant-id $AZURE_TENANT_ID --federated-credential-provider oidc
    - azd auth login --check-status