WIP: compute-matrix: add 'docs-build' matrix, switch docs builds to RTX Pro 6000

[jameslamb]

Introduces a docs-build matrix to the compute-matrix job, so we can centrally control which runner types are used for docs builds.

Details

RAPIDS projects generally need a GPU in their docs builds because things like Sphinx autosummary require actually import-ing the libraries, and many of the libraries require a GPU at runtime.

Today, projects are using the custom-job shared workflow and hard coding a GPU type, mostly L4.

  docs-build:
    uses: rapidsai/shared-workflows/.github/workflows/custom-job.yaml@main
    if: fromJSON(needs.changed-files.outputs.changed_file_groups).build_docs
    with:
      build_type: pull-request
      node_type: "gpu-l4-latest-1"
      arch: "amd64"
      container_image: "rapidsai/ci-conda:26.10-latest"
      script: "ci/build_docs.sh"

(example from NVIDIA/cuml)

L4 runners are heavily used in the shared CI runner system we use right now, which can result in long queue lines:

Look how often they've been 100% utilized in the last 2 days:

We'd like to switch to a less-busy runner type, such as RTX Pro 6000:

Centralizing that choice of runner type would mean that changing it in the future wouldn't require PRs like NVIDIA/cudf#23878 in every repo. I think that's justified here... it's really unlikely that docs builds would be sensitive to the particular details of a runner, as long as it's a GPU RAPIDS supports.