Fix docs-build-pr not triggering on elastic/docs PRs (#3332)

The docs-build-pr pipeline has a branch filter in Buildkite that only
allows master. All other repos in job 0 use always_trigger_branch:master
so they always build against master and pass the filter. The standalone
elastic/docs job (job 1) has no always_trigger_branch, so the pr-bot
triggers against the PR's feature branch which the pipeline filter
silently rejects.

Previously builds appeared to work because the doc-preview GitHub Actions
workflow posts a comment containing the text "run docs-build", which
accidentally matched the old unanchored trigger_comment_regex. Tightening
the regex with ^ and $ anchors (#3327) inadvertently removed that
accidental trigger.

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

Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: Mpdreamz

.buildkite/pull-requests.org-wide.json

@@ -91,6 +91,7 @@
       "allowed_list": ["github-actions[bot]", "renovate[bot]", "mergify[bot]"],
       "build_on_commit": true,
       "build_on_comment": true,
+      "ignore_pipeline_branch_filters": true,
       "trigger_comment_regex": "^run docs-build ?(?<rebuild_opt>rebuild)? ?(?<warn_opt>warnlinkcheck)? ?(?<skip_opt>skiplinkcheck)?\\s*?$",
       "always_trigger_comment_regex": "^buildkite test this ?(?<rebuild_opt>rebuild)? ?(?<warn_opt>warnlinkcheck)? ?(?<skip_opt>skiplinkcheck)?\\s*?$",
       "skip_ci_labels": [