docs: document az-sync action and nginx.org-make-aws workflow

[Copilot]

Proposed changes

Documents two files added in the nginx.org branch that had no README coverage: the az-sync composite action and the nginx.org-make-aws reusable workflow.

az-sync/action.yml

• Inputs table (Azure OIDC credentials, Key Vault name, secrets filter pattern)
• Usage example showing how to call the action from a workflow step
• Note on multiline secret handling via $GITHUB_ENV heredoc syntax

nginx.org-make-aws.yml

• Description of the two-job staging-then-promote pattern (build-staging → build-prod via S3 marker)
• Required GitHub secrets and Key Vault secrets tables
• Inputs table with defaults
• Access control rules (org / event / ref / actor restrictions enforced at runtime)
• Caller example for referencing the workflow from another repo

All existing documentation is left intact.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

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