add enable-extensible-builders to Java configration (#366)

Author: tstanmay13

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

@@ -44,6 +44,11 @@ Example:
 <ParamField path="disable-required-property-builder-checks" type="boolean" default="false" required={false} toc={true}>
 </ParamField>
 
+<ParamField path="enable-extensible-builders" type="boolean" default="false" required={false} toc={true}>
+
+When enabled, generates extensible builder classes that support customization through inheritance. This allows you to [add custom code](/learn/v2/sdks/generators/java/custom-code#adding-custom-client-configuration) to extend the generated builders with additional functionality.
+</ParamField>
+
 <ParamField path="enable-forward-compatible-enums" type="boolean" default="false" required={false} toc={true}>
 </ParamField>