docs: add x-jsonschema-if extension page

registries/_extension/x-jsonschema-if.md

@@ -0,0 +1,48 @@
+---
+owner: baywet
+issue:
+description: The JSON Schema if conditional subschema, used when targeting OpenAPI versions that do not directly support it.
+schema:
+  $ref: "#/$defs/schemaObject"
+objects: [ "Schema Object" ]
+layout: default
+---
+
+{% capture summary %}
+The JSON Schema `if` keyword defines a conditional subschema that controls whether `then` or `else` applies.
+
+The `x-jsonschema-if` extension mirrors the JSON Schema `if` keyword when targeting OpenAPI versions where the keyword is not directly available. It is serialized as `x-jsonschema-if` so tools can preserve and process the conditional subschema.
+
+It can appear as a property in the following objects: `{{page.objects|jsonify}}`.
+
+Used by: (informational)
+
+* [Microsoft.OpenApi](https://github.com/microsoft/OpenAPI.NET) (.NET OpenAPI library)
+{% endcapture %}
+
+{% capture example %}
+```yaml
+openapi: 3.0.4
+info:
+  title: My API
+  version: 1.0.0
+paths: {}
+components:
+  schemas:
+    Address:
+      type: object
+      properties:
+        country:
+          type: string
+        postalCode:
+          type: string
+      x-jsonschema-if:
+        properties:
+          country:
+            const: US
+        required:
+          - country
+```
+{% endcapture %}
+
+{% include extension-entry.md summary=summary example=example %}