[ci] switch to shared 'docs-build' matrix?

[jameslamb]

Description

I recently did some work across RAPIDS to:

1. switch docs builds from L4 runners to RTX Pro 6000 (which we have a lot more free capacity for)
2. centralize that decision in a shared matrix, so it's easy to do something like "switch all docs builds to a different runner type" in the future

Details here: rapidsai/build-planning#333

I skipped cuopt because its main branch pointed at release/26.10 on shared-workflows and that made this change a bit awkward.

This project should consider a change like that too, when you update to main of shared-workflows.

Benefits of this work

• reduced time spent waiting for docs builds to get scheduled
• reduced load on NVIDIA's shared and oversubscribed L4 runners
• RAPIDS build engineering team manages the runner type for docs builds so you don't need to actively manage it

Approach

Imitate the PRs from rapidsai/build-planning#333