docs: add health service

Signed-off-by: Vincenzo Scamporlino <vincenzos@spotify.com>

docs/backend-system/core-services/01-index.md

@@ -20,6 +20,7 @@ import { coreServices } from '@backstage/backend-plugin-api';
 - [Cache Service](./cache.md) - Key-value store for caching data.
 - [Database Service](./database.md) - Database access and management via [knex](https://knexjs.org/).
 - [Discovery Service](./discovery.md) - Service discovery for inter-plugin communication.
+- [Health Service](./health.md) - Health check endpoints for the backend.
 - [Http Auth Service](./http-auth.md) - Authentication of HTTP requests.
 - [Http Router Service](./http-router.md) - HTTP route registration for plugins.
 - [Identity Service](./identity.md) - Deprecated user authentication service, use the [Auth Service](./auth.md) instead.

docs/backend-system/core-services/health.md

@@ -0,0 +1,39 @@
+---
+id: health
+title: Heath Service
+sidebar_label: Health
+description: Documentation for the Health service
+---
+
+The Health service provides some health check endpoints for the plugins. By default, it attaches `/.backstage/health/v1/readiness` and `/.backstage/health/v1/liveness` endpoints to the backend server, which return a JSON object with the status of the backend services.
+
+## Configuring the service
+
+The following example is how you can override the health service to add custom endpoints.
+
+```ts
+import { coreServices } from '@backstage/backend-plugin-api';
+import { WinstonLogger } from '@backstage/backend-app-api';
+
+const backend = createBackend();
+
+backend.add(
+  createServiceFactory({
+    service: coreServices.health,
+    deps: {
+      rootHttpRouter: coreServices.rootHttpRouter,
+    },
+    async factory({ rootHttpRouter }) {
+      rootHttpRouter.get('.backstage/health/v1/readiness', async (req, res) => {
+        res.json({ status: 'ok' });
+      });
+
+      rootHttpRouter.get('.backstage/health/v1/liveness', async (req, res) => {
+        res.json({ status: 'ok' });
+      });
+
+      return {};
+    },
+  }),
+);
+```