Troubleshoot Observability cleanup (#473)

Cleans up 'align serverless/stateful' pages in the Troubleshoot >
Observability section.
Removes raw migrated files for the cleaned-up pages if they're not used
elsewhere.

Previews:

-
[Synthetics](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/473/troubleshoot/observability/troubleshooting-synthetics)
-
[Logs](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/473/troubleshoot/observability/troubleshoot-logs)
- [Common
problems](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/473/troubleshoot/observability/apm/common-problems)
-
[SLOs](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/473/troubleshoot/observability/troubleshoot-service-level-objectives-slos)

Author: natasha-moore-elastic