Issue #25179: Documentation for formatted string literals aka f-strings

Some of the inspiration and wording is taken from the text of PEP 498 by Eric
V. Smith, and the existing str.format() documentation.

Doc/tutorial/inputoutput.rst

@@ -25,7 +25,8 @@ first way is to do all the string handling yourself; using string slicing and
 concatenation operations you can create any layout you can imagine.  The
 string type has some methods that perform useful operations for padding
 strings to a given column width; these will be discussed shortly.  The second
-way is to use the :meth:`str.format` method.
+way is to use :ref:`formatted string literals <f-strings>`, or the
+:meth:`str.format` method.
 
 The :mod:`string` module contains a :class:`~string.Template` class which offers
 yet another way to substitute values into strings.

Doc/tutorial/introduction.rst

@@ -352,6 +352,9 @@ The built-in function :func:`len` returns the length of a string::
       Strings support a large number of methods for
       basic transformations and searching.
 
+   :ref:`f-strings`
+      String literals that have embedded expressions.
+
    :ref:`formatstrings`
       Information about string formatting with :meth:`str.format`.