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
• reference/observability/index.md
• solutions/observability.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

🔔 The preview site may take up to 3 minutes to finish building. These links will become live once it completes.

[theletterf]

@mdbirnstiehl Thanks for the excellent review! I've applied most changes and refreshed several screenshots.

As for the collect data step, you're right, though in general we prefer folks to start with infra to fully harness infra-apm correlation. I've added a small non-callout note though.

Edit: I've also edited the main Observability index, to make it more about where to find each piece of the Obs docs and less about Get started, as it would have been redundant.

[mdbirnstiehl]

🚀 looks great! nice work!

[leemthompo]

Joining the ✅ party

[yetanothertw]

It has so many reviewers!! Here's the rubber stamp :)