gh-106318: Add doctest role and 'See also' for str.zfill()

Author: adorilson

Doc/library/stdtypes.rst

@@ -2842,13 +2842,17 @@ expression support in the :mod:`re` module).
    than before. The original string is returned if *width* is less than
    or equal to ``len(s)``.
 
-   For example::
+   For example:
+
+   .. doctest::
 
       >>> "42".zfill(5)
       '00042'
       >>> "-42".zfill(5)
       '-0042'
 
+   See also :meth:`rjust`.
+
 
 .. index::
    single: ! formatted string literal