Consolidate route configuration

[CoenWarmer]

Part of #152783

📝 Summary

The route configuration for the Observability app is stored in x-pack/plugins/observability/public/routes/index.tsx. In x-pack/plugins/observability/public/config/paths.ts there are also paths configured separately which are used throughout the app.

Lets bring them closer together so it's not possible to accidentally change one without changing the other.

In addition, lets use the same route path constants everywhere in the app.

✅ Acceptance criteria

• Route config should be stored in one place
• Constants for routes should be reused everywhere in the app

[elasticmachine]

Pinging @elastic/actionable-observability (Team: Actionable Observability)