add Lambda to API reference

docs/source/api/codegen.md

@@ -177,4 +177,5 @@ The following categories with target-specific options are exposed:
   Kernel
   GpuKernel
   Parameter
+  Lambda
 ```

src/pystencils/codegen/__init__.py

@@ -6,6 +6,7 @@ from .config import (
 from .parameters import Parameter
 from .kernel import Kernel, GpuKernel
 from .driver import create_kernel, get_driver
+from .functions import Lambda
 
 __all__ = [
     "Target",
@@ -14,6 +15,7 @@ __all__ = [
     "Parameter",
     "Kernel",
     "GpuKernel",
+    "Lambda",
     "create_kernel",
     "get_driver",
 ]