docs: add health check, readiness, and liveness endpoints guide (#281)

Document the liveness, readiness, and detailed health probes exposed by
self-hosted Plane (Commercial Edition) services, and how to consume them.

- New page: self-hosting/manage/health-checks.md covering the primary API
  probes (/api/live/, /api/ready/, /api/health/), per-service endpoints
  (pi, live, silo, flux, node-runner) and the Go monitor prober, with curl
  examples, exact JSON responses, status codes, and the 5s result cache.
- Add "use in your infrastructure" examples: Kubernetes liveness/readiness
  probes, Docker Compose healthchecks, and external uptime/LB guidance,
  based on Plane's actual Helm and Compose probe configs.
- Note the edition split: Community Edition exposes only the root / check;
  the dedicated probes are Commercial Edition.
- Wire the page into the Self-hosting > Manage sidebar.

Author: sriramveeraghanta

docs/.vitepress/config.mts

@@ -386,6 +386,7 @@ export default withMermaid(
               { text: "Upgrade Community to Commercial Edition", link: "/self-hosting/upgrade-from-community" },
               { text: "Upgrade Community to Airgapped Edition", link: "/self-hosting/manage/community-to-airgapped" },
               { text: "View Logs", link: "/self-hosting/manage/view-logs" },
+              { text: "Health checks", link: "/self-hosting/manage/health-checks" },
               { text: "Migrate Plane", link: "/self-hosting/manage/migrate-plane" },
               { text: "Prime CLI", link: "/self-hosting/manage/prime-cli" },
               { text: "Manage users", link: "/self-hosting/manage/manage-instance-users" },