Draft an extra x-topic for versioning

docs/overlays/elasticsearch-serverless-openapi-overlays.yaml

@@ -44,4 +44,54 @@ actions:
     description: Add document security
     update:
       security:
-        - apiKeyAuth: []
+        - apiKeyAuth: []
+# Add x-topics
+  - target: '$'
+    description: Add an extra page about versioning
+    update:
+      x-topics:
+        - title: API versioning
+          content: |
+            Elasticsearch Serverless implements API versioning to enable backwards-incompatible changes while maintaining client compatibility. An API version is a date-based identifier that represents a guaranteed backwards-compatible API surface.
+
+            As long as the server supports an API version and the client specifies that version, existing client code will continue to work without modification. This backwards compatibility guarantee covers request/response structure and API behavior.
+
+            ## Guarantees
+
+            This API versioning scheme provides several key guarantees:
+
+            * No code changes required: Your application code remains unchanged during Serverless infrastructure upgrades.
+            * No manual migrations: Data, indices, and resources are migrated transparently without user intervention.
+            * New features available everywhere: Compatible additions like new APIs and parameters work across all supported versions.
+            * Predictable upgrade timeline: API versions are supported for at least 18 months after newer versions are introduced.
+
+            ## API version format
+
+            API versions use ISO8601 date format: `YYYY-MM-DD` (for example, `2023-10-31`).
+
+            ## Supported versions
+
+            Elasticsearch Serverless currently supports the following API versions:
+
+            * `2023-10-31`
+
+            Future breaking changes will introduce new date-based versions.
+            Support policies for existing versions will be announced when new versions are released.
+
+            ## How to specify API versions
+
+            Clients specify API versions using the `Elastic-Api-Version` HTTP header:
+
+            ```
+            GET /_search HTTP/1.1
+            Elastic-Api-Version: 2023-10-31
+            ```
+
+            ### Server responses
+
+            The server's response depends on whether the client provides a version header:
+
+            * When a client sends the `Elastic-Api-Version` header, the server processes the request using the specified API version or returns HTTP 400 if the version is unsupported or malformed. 
+            * When no version header is provided, the server defaults to the latest supported API version.
+
+            The server always includes the Elastic-Api-Version header in successful responses to indicate which version was used.