docs-build: make use of 'cpu8' runners explicit

[jameslamb]

Contributes to rapidsai/build-planning#333

Updates to hopefully reduce how often docs-build jobs get stuck waiting for CI runners.

* switches those jobs to RTX Pro 6000 runners
* uses shared docs-build matrix to set job details

After initially opening this to switch the type of GPU runner used (as I'm doing across the rest of RAPIDS), I realize that this repo's actually using CPU-only runners for its docs builds.

That wasn't obvious because it's implicitly relying on the default value here: https://github.com/rapidsai/shared-workflows/blob/e9a2657b6a56aae9f8fe2ef4bc8e613fd79438d4/.github/workflows/custom-job.yaml#L41

This PR makes that explicit.

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Confidence Score: 5/5

The PR appears safe to merge with no substantive issues identified.

Summary

• Keeps the existing AMD64 architecture and CI container.
• Makes CPU runner selection explicit for the shared custom-job workflow.

_{Reviews (2) · Last reviewed commit: "make use of CPU runners explicit"}

[jameslamb]

/merge