Clean up observability apps section

[colleenmcginnis]

Cleans up observability apps section including:

• In content that applies to both stateful and serverless, use APM Server or the managed intake service instead of APM Server as there is no APM Server in serverless.
• Add missing serverless content (e.g. getting started, api keys).
• Update syntax that leads to rendering issues.
• Simplify internal links.
• Clean up unnecessary white space.

[colleenmcginnis]

Note: I'm not creating a redirect here because in the final list of redirects for the web team, we're redirecting https://www.elastic.co/guide/en/observability/current/apm-performance-diagnostic.html to troubleshoot/observability/apm/apm-server-performance-diagnostic.md.

[colleenmcginnis]

Note: I'm not creating a redirect here because in the final list of redirects for the web team, we're redirecting https://www.elastic.co/guide/en/observability/current/apm-common-response-codes.html to troubleshoot/observability/apm/apm-server-response-codes.md.

[colleenmcginnis]

Note: I'm not creating a redirect here because in the final list of redirects for the web team, we're redirecting https://www.elastic.co/guide/en/observability/current/apm-enable-apm-server-debugging.html to
troubleshoot/observability/apm/enable-apm-server-binary-debugging.md.

[colleenmcginnis]

Note: I'm not creating a redirect here because in the final list of redirects for the web team, we're redirecting https://www.elastic.co/guide/en/observability/current/apm-processing-and-performance.html to troubleshoot/observability/apm/processing-performance.md.

[colleenmcginnis]

Note: I'm not creating a redirect here because in the final list of redirects for the web team, we're redirecting https://www.elastic.co/guide/en/observability/current/apm-server-es-down.html to troubleshoot/observability/apm/what-happens-when-apm-server-elasticsearch-is-down.md.

[mdbirnstiehl]

Just a couple of small things, otherwise LGTM! 🦭