gh-157208: Fix the ctypes.util.struct documentation

The anonymous-field example used Annotated, CFieldInfo, c_ssize_t and
c_void_p without importing any of them, so it raised NameError when
copied out of the page. Add the imports and indent the example body
four spaces to match the example above it.

The signature line showed layout without its default, rendering an
optional parameter as required.

Author: fedonman

Doc/library/ctypes.rst

@@ -3207,7 +3207,7 @@ fields, or any other data types containing pointer type fields.
       that should be merged into a containing structure or union.
 
 
-.. decorator:: struct(*, align=None, layout, endian='native', pack=None)
+.. decorator:: struct(*, align=None, layout=None, endian='native', pack=None)
    :module: ctypes.util
 
    A :term:`decorator` that allows generating structure types using an
@@ -3244,14 +3244,18 @@ fields, or any other data types containing pointer type fields.
 
    .. code-block:: python
 
+      from typing import Annotated
+      from ctypes import c_ssize_t, c_void_p
+      from ctypes.util import struct, CFieldInfo
+
       @struct
       class PyObject:
-         ob_refcnt: c_ssize_t
-         ob_type: c_void_p
+          ob_refcnt: c_ssize_t
+          ob_type: c_void_p
 
       @struct
       class PyHovercraftObject:
-         ob_base: Annotated[PyObject, CFieldInfo(anonymous=True)]
+          ob_base: Annotated[PyObject, CFieldInfo(anonymous=True)]
 
    .. versionadded:: next