Docs: add bridge jobs section to downstream pipelines

What does this MR do?

Adds a short Bridge jobs section to the downstream pipelines docs, defining what a bridge job (trigger job) is and explaining its pending lifecycle.

The page previously never defined the term "bridge job", even though it's used throughout the GitLab backend and UI. The new section clarifies that a long-running pending bridge job is not a runner availability problem, but usually means the downstream pipeline could not be created.

Related

Relates to Bridge jobs state is "permanently" pending (#602278).

A follow-up MR will add a dedicated troubleshooting section to downstream_pipelines_troubleshooting.md, and at that point this bridge jobs section will be updated to link to it more specifically.