Remove "Collector in gateway mode" terminology from docs

Craig flagged to me that "gateway mode" is a misnomer. A gateway is a specific configuration of a collector that accepts data from other collectors (a deployment pattern), and not a distinct mode.

To do: audit all docs for references to "gateway mode" and update them to reflect the correct framing (e.g., "gateway deployment," "gateway configuration," or "gateway pattern"). Ensure no docs imply it's a separate operational mode alongside other modes.