Fix docs-build-pr not triggering on elastic/docs PRs

[Mpdreamz]

Problem

The docs-build-pr Buildkite pipeline has a branch filter configured to only allow master. All other repos use job 0 which has always_trigger_branch: master — so builds always trigger against the master branch and pass the filter.

The standalone elastic/docs entry (job 1) has no always_trigger_branch, so the pr-bot triggers against the PR's feature branch, which the pipeline filter silently rejects. No build, no GitHub check.

Note: This change affects elastic/docs only. All other repositories (elastic/elasticsearch, elastic/kibana, etc.) use a separate job (job 0) with always_trigger_branch: master and are not affected by this change.

Why it appeared to work before

Previously, builds appeared to trigger because the doc-preview GitHub Actions workflow posts a comment on every new PR containing the text `run docs-build`. The old trigger_comment_regex had no ^/$ anchors, so it matched any comment containing "run docs-build" — accidentally triggering a build via the comment path.

Tightening the regex with anchors in #3327 correctly stopped that partial match, but also removed the only mechanism that was making it work.

Fix

Add ignore_pipeline_branch_filters: true to job 1. This tells the Buildkite API to bypass the pipeline's branch filter when triggering, allowing build_on_commit and comment triggers to work as intended per the buildkite-pr-bot design.

Security is still enforced by allow_org_users: true and allowed_repo_permissions: [admin, write] — external contributors cannot trigger builds.

Testing

After merge, verify that PR #3331 (test/buildkite-docs-build-pr-trigger) shows a buildkite/docs-build-pr check on the next push or buildkite test this comment.

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