Rebuild onboarding docs (Get Started) for obs

[theletterf]

This PR:

• Rebuilds the Get started structure for Obs around the main onboarding flow as a supertask.
• Migrates most of the Obs content from https://www.elastic.co/getting-started/
• Moves out some guides that don't belong to Get started (Add Splunk data, Java tutorial, and so on).
• Adds relevant metadata to existing Quickstarts and give them a secondary role.

Each step of the supertask should link to relevant day-2 docs within Observability or to EDOT reference docs.

Pending: Moving EDOT narrative docs to Solutions.

[github-actions]

🔍 Preview links for changed docs:

• deploy-manage/deploy/elastic-cloud/create-serverless-project.md
• deploy-manage/deploy/elastic-cloud/serverless.md
• manage-data/ingest/ingest-reference-architectures/agent-to-es.md
• solutions/observability/apm/get-started-serverless.md
• solutions/observability/applications/index.md
• solutions/observability/get-started.md
• solutions/observability/get-started/other-tutorials/add-data-from-splunk.md
• solutions/observability/get-started/other-tutorials/index.md
• solutions/observability/get-started/other-tutorials/tutorial-monitor-java-application.md
• solutions/observability/get-started/quickstart-collect-data-with-aws-firehose.md
• solutions/observability/get-started/quickstart-create-synthetic-monitor.md
• solutions/observability/get-started/quickstart-elastic-cloud-otel-endpoint.md
• solutions/observability/get-started/quickstart-monitor-hosts-with-elastic-agent.md
• solutions/observability/get-started/quickstart-monitor-hosts-with-opentelemetry.md
• solutions/observability/get-started/quickstart-monitor-kubernetes-cluster-with-elastic-agent.md
• solutions/observability/get-started/quickstart-monitor-your-application-performance.md
• solutions/observability/get-started/quickstart-unified-kubernetes-observability-with-elastic-distributions-of-opentelemetry-edot.md
• solutions/observability/get-started/quickstarts.md

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