ci(docs): give each PR its own docs concurrency group

[bburda]

Pull Request

Summary

All Documentation runs used one concurrency group, pages. When a third run joined the group, GitHub cancelled the waiting one, so a PR could end up with no docs build.

• PR runs now use one group per PR. A new push cancels only the older build of the same PR.
• Runs on main share one group, and a running build on main is never cancelled.
• The deploy job keeps the pages group, so only one Pages deployment runs at a time.

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Issue

• closes [BUG] Docs builds of one PR get cancelled by docs builds of other PRs #716

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Type

• Bug fix
• New feature or tests
• Breaking change
• Documentation only

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Testing

• actionlint .github/workflows/docs.yml passes.
• While this PR's docs build runs, start the workflow by hand on the branch: gh workflow run docs.yml --ref ci/docs-concurrency. Both builds run at the same time and both finish. With the old pages group, the second build waits for the first one.

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Checklist

• Breaking changes are clearly described (and announced in docs / changelog if needed)
• Tests were added or updated if needed
• Docs were updated if behavior or public API changed