Key the workflow template concurrency group on the pull request number

[albertvillanova]

What does this PR do?

The build_pr_documentation.yml template keys its concurrency group on github.head_ref, which is the head branch name without the fork owner. Two open PRs from different forks whose branches share a name therefore land in the same group, and a push on one cancels the in-flight doc build on the other.

Shared names are common, because many contributors push to their fork's default branch instead of creating one: in transformers, 154 of the 1589 currently open PRs share a head branch name with another open PR, 84 of them on main and 41 on patch-1. I did not find a realized cancellation in the 1200 most recent runs of that doc build, so this is hardening rather than a fix for something currently failing.

github.event.pull_request.number is unique per pull request. The template only triggers on pull_request, so the github.run_id fallback is never reached, but keeping it means that a consumer who later adds another trigger falls back to never cancelling rather than to grouping by branch name.

The snippet has been copied widely: 41 of the 42 repositories in the organization that have a build_pr_documentation.yml carry this line byte for byte, and it has also been copied into test and benchmark workflows, where a cancelled run costs runner minutes rather than a doc preview.

It also drops the reference to the datasets library from the sentence introducing the snippet. That claim did not hold anyway: datasets pins the uses field to a commit SHA while the snippet shows @main, so the section reads better as a template than as a copy of one repository's workflow.