Update configuration.mdx based on issue #266

Author: kgowru

fern/products/sdks/overview/python/configuration.mdx

@@ -15,7 +15,30 @@ groups:
         config:
           package_name: "your_package"
           client:
-            class_name: "YourClient" 
+            class_name: "YourClient"
+```
+
+## Extension Headers
+
+You can configure SDK variables in your OpenAPI spec using the `x-fern-sdk-variables` extension. These variables will be injected into the SDK client constructor.
+
+For example, to add a `project_id` variable that gets automatically injected into path parameters:
+
+```yaml
+components:
+  x-fern-sdk-variables:
+    project_id:
+      type: string
+      description: The ID of the project
+      pattern: "^proj_[a-zA-Z0-9]+$"
+```
+
+The variable will be available in the client constructor:
+
+```python
+from my_package import Client
+
+client = Client(project_id="proj_123")
 ```
 
 ## SDK Configuration Options
@@ -72,7 +95,6 @@ groups:
 </ParamField>
 
 <ParamField path="package_name" type="string" default="null" required={false} toc={true}>
-
 Specifies the Python package name that users will import your generated client
 from.
 
@@ -112,8 +134,8 @@ groups:
   # Visit every case in the union
   shape = get_shape()
   shape.visit(
-    circle: lambda circle: do_something_with_circle(circle),
-    triangle: lambda triangle: do_something_with_triangle(triangle),
+    circle=lambda circle: do_something_with_circle(circle),
+    triangle=lambda triangle: do_something_with_triangle(triangle),
   )
   ```