update deployment actions to use GitHub's native concurrency key

[kgraeper]

This serializes deployments per repo+ref using GitHub's built-in concurrency mechanism, which works correctly regardless of whether the workflow is invoked directly or via workflow_call. Using cancel-in-progress: false ensures queued deployments wait rather than being cancelled (matching turnstyle's original queue-and-wait behavior).

[Copilot]

Pull request overview

This PR updates the documentation deployment workflow to serialize GitHub Pages deployments using GitHub Actions’ native concurrency mechanism, and removes the previous softprops/turnstyle@v3-based serialization from the composite deploy action (with explanatory documentation).

Changes:

• Add a concurrency key to the docs-deploy job to queue deployments instead of canceling them.
• Remove the softprops/turnstyle@v3 step from deploy-versioned-pages composite action.
• Document why turnstyle fails under workflow_call and how to configure native concurrency instead.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
.github/workflows/docs.yml	Adds job-level GitHub Actions concurrency to serialize Pages deployments.
.github/actions/deploy-versioned-pages/action.yml	Removes turnstyle step and adds documentation guiding callers to use native concurrency.

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