pythongh-106318: Add example for str.find() (python#134529)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: blaisep

Doc/library/stdtypes.rst

@@ -1911,6 +1911,14 @@ expression support in the :mod:`re` module).
    Return the lowest index in the string where substring *sub* is found within
    the slice ``s[start:end]``.  Optional arguments *start* and *end* are
    interpreted as in slice notation.  Return ``-1`` if *sub* is not found.
+   For example::
+
+      >>> 'spam, spam, spam'.find('sp')
+      0
+      >>> 'spam, spam, spam'.find('sp', 5)
+      6
+
+   See also :meth:`rfind` and :meth:`index`.
 
    .. note::