Fix outdated names and add APM troubleshooting on OpenTelemetry pages

[brett0000FF]

What does this PR do? What is the motivation?

Small accuracy fixes across the OpenTelemetry docs, plus one new troubleshooting entry.

• Uses "upstream" instead of "standalone" for the open source OpenTelemetry Collector. Elsewhere in the docs, "standalone" describes the standalone DDOT Collector, so the same word meant two different things.
• Rewords the host metrics receiver note: the receiver isn't needed when the DDOT Collector runs with the Datadog Agent. The old wording said it was needed only for the upstream Collector, which doesn't hold for the standalone DDOT Collector.
• Replaces names that no longer match the docs: a compatibility table column ("OTel to Datadog Agent (OTLP)"), "Datadog Tracer," and "Service Catalog."
• Removes the compatibility table pointer from OTLP Ingest in the Agent. The table has no column for that setup.
• Adds "Services or trace metrics are missing in APM" to Troubleshooting, with the fix for each setup.

Merge readiness

• Ready for merge

AI assistance

Drafted with Claude Code. Changes were reviewed by hand against the docs style guide.

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Additional notes

Requesting a technical review of the new troubleshooting entry, especially whether the DDOT Collector or OTLP Ingest in the Agent have their own causes for this symptom.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/brett.blue/otel-docs-fixes/opentelemetry/instrument/dd_sdks/instrumentation_libraries
• https://docs-staging.datadoghq.com/brett.blue/otel-docs-fixes/opentelemetry/instrument/dd_sdks/otlp_trace_export
• https://docs-staging.datadoghq.com/brett.blue/otel-docs-fixes/opentelemetry/integrations/host_metrics
• https://docs-staging.datadoghq.com/brett.blue/otel-docs-fixes/opentelemetry/migrate/ddot_collector
• https://docs-staging.datadoghq.com/brett.blue/otel-docs-fixes/opentelemetry/migrate/migrate_operation_names
• https://docs-staging.datadoghq.com/brett.blue/otel-docs-fixes/opentelemetry/setup/collector_exporter/
• https://docs-staging.datadoghq.com/brett.blue/otel-docs-fixes/opentelemetry/setup/otlp_ingest_in_the_agent
• https://docs-staging.datadoghq.com/brett.blue/otel-docs-fixes/opentelemetry/troubleshooting