[docs-patrol] Update CI pipeline docs for first backport-branch push behavior

[github-actions]

Recent code changes introduced documentation drift in CI pipeline behavior.

Changes Requiring Documentation Updates

1. First push to backport-* branches now skips package-test diffing baseline behavior described in docs

Commit(s):

• 7ed2346

What changed:

• .buildkite/scripts/common.sh updated get_from_changeset() so that when there is no previous successful build and branch matches backport-*, it sets from="\$\{BUILDKITE_COMMIT}" instead of origin/\$\{BUILDKITE_BRANCH}^.
• This special-cases the first push of a new backport branch and avoids selecting all packages for testing.

Documentation impact:

• docs/ci_pipelines.md still states that for branch context (main or backport-*), CI always diffs against the latest successful build and then selects packages from that change set (docs/ci_pipelines.md lines 71-73).
• It does not document the first-backport-push exception added in the commit above.

Suggested Actions

• Update docs/ci_pipelines.md branch-context section to describe the first-push backport-* exception when no previous successful build exists.
• Link the behavior to the relevant script path (.buildkite/scripts/common.sh) so future CI behavior changes are easier to track in docs.

---

What is this? | From workflow: Docs Patrol

Give us feedback! React with 🚀 if perfect, 👍 if helpful, 👎 if not.

• expires on May 19, 2026, 10:56 AM UTC