Key the doc build concurrency group on the pull request number

[albertvillanova]

What does this PR do?

build_pr_documentation.yml keys its concurrency group on github.head_ref, which is the head branch name without the fork owner. Two open PRs from different forks that both use a branch called patch-1 therefore land in the same group, and a push on one cancels the in-flight doc build on the other.

github.event.pull_request.number is unique per pull request. This workflow only triggers on pull_request, so the github.run_id fallback is never reached, but it is kept so the block stays identical to the one in tests.yml and tests-experimental.yml. Happy to drop it if you would rather the expression say only what this workflow needs.

Bugbot found the same pattern on #7170 (comment), where it had been copied from here. No two of the 182 currently open PRs share a head branch name, but 130 of them come from forks, so the collision is reachable.

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Note

Low Risk
Single CI workflow concurrency expression change with no runtime or application impact.

Overview
Fixes cross-PR doc build cancellations when multiple forks use the same branch name (e.g. patch-1).

The Build PR Documentation workflow’s concurrency.group now uses github.event.pull_request.number instead of github.head_ref, so each PR gets its own group and pushes on one PR no longer cancel in-flight doc builds on another. The github.run_id fallback is unchanged for parity with other workflows.

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[bot-ci-comment]

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[albertvillanova]

build_pr_documentation.yml was itself copied from the workflow template in the doc-builder README, which still carries the head_ref form. The same one-liner is proposed there in huggingface/doc-builder#829, so new copies stop inheriting it.