pythongh-106318: Add examples for str.startswith() method (python#144369)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>

Author: adorilson

Doc/library/stdtypes.rst

@@ -2751,6 +2751,19 @@ expression support in the :mod:`re` module).
    test string beginning at that position.  With optional *end*, stop comparing
    string at that position.
 
+   For example:
+
+   .. doctest::
+
+      >>> 'Python'.startswith('Py')
+      True
+      >>> 'a tuple of prefixes'.startswith(('at', 'a'))
+      True
+      >>> 'Python is amazing'.startswith('is', 7)
+      True
+
+   See also :meth:`endswith` and :meth:`removeprefix`.
+
 
 .. method:: str.strip(chars=None, /)