docs: split OpenTelemetry export into its own page under gateway

Logging.md had grown to 487 lines with ~300 lines dedicated to
OpenTelemetry export — wire protocol, full metric/span catalog, env
vars, captureContent shape, sampling, the diagnostic event catalog,
and protocol notes — leaving the genuine logging overview buried
behind exporter reference material.

Move the OTEL surface to a dedicated page and slim logging.md to a
focused logs overview:

- Add docs/gateway/opentelemetry.md (OpenTelemetry export). Same
  content reorganized: how it fits together, quick start, signals,
  configuration reference + env vars table, privacy/captureContent,
  sampling/flushing, full metric and span catalog, diagnostic event
  catalog, no-exporter mode, diagnostics flags pointer, disable.
- docs/logging.md: drop the OTEL section in favor of a short
  'Diagnostics and OpenTelemetry' summary that cross-links the new
  page and the diagnostics-flags page. Drops 273 lines net. Also
  drops the redundant body H1, retitles to 'Logging' (was 'Logging
  overview' which mismatched sidebar usage), and refreshes the
  Related list.
- docs/docs.json: insert gateway/opentelemetry into the
  'Health and diagnostics' sidebar group, reorder pages so the user-
  facing health/run pages come before exporter/internals pages, and
  put logging next to opentelemetry where readers naturally
  associate them.
- docs/gateway/diagnostics.md, docs/gateway/logging.md,
  docs/gateway/configuration-reference.md: cross-link the new page
  and sentence-case stale Title-Cased Related entries on
  diagnostics.md.

docs/gateway/diagnostics.md

@@ -129,9 +129,10 @@ diagnostic event collection:
 Disabling diagnostics reduces bug-report detail. It does not affect normal
 Gateway logging.
 
-## Related docs
+## Related
 
-- [Health Checks](/gateway/health)
+- [Health checks](/gateway/health)
 - [Gateway CLI](/cli/gateway#gateway-diagnostics-export)
-- [Gateway Protocol](/gateway/protocol#system-and-identity)
+- [Gateway protocol](/gateway/protocol#system-and-identity)
 - [Logging](/logging)
+- [OpenTelemetry export](/gateway/opentelemetry) — separate flow for streaming diagnostics to a collector