Rewrite APM get started docs

[theletterf]

Affected docs:

• https://www.elastic.co/docs/solutions/observability/apm/get-started
• https://www.elastic.co/docs/solutions/observability/apm/get-started-serverless
• https://www.elastic.co/docs/solutions/observability/apm/use-opentelemetry-with-apm

Actions:

• Recreate diagrams for OTel ingest
• Reflect new onboarding recommended paths