Add documentation examples of consider-using-f-string

doc/data/messages/c/consider-using-f-string/bad.py

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+from string import Template
+menu = ('eggs', 'spam', 42.4)
+
+old_order = "%s and %s: %.2f ¤" % menu # [consider-using-f-string]
+beginner_order = menu[0] + " and " + menu[1] + ": " + str(menu[2]) + " ¤"
+joined_order = " and ".join(menu[:2])
+format_order = "{} and {}: {:0.2f} ¤".format(menu[0], menu[1], menu[2]) # [consider-using-f-string]
+named_format_order = "{eggs} and {spam}: {price:0.2f} ¤".format(eggs=menu[0], spam=menu[1], price=menu[2]) # [consider-using-f-string]
+template_order = Template('$eggs and $spam: $price ¤').substitute(eggs=menu[0], spam=menu[1], price=menu[2])

doc/data/messages/c/consider-using-f-string/details.rst

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+Formatted string literals (f-strings) give a concise, consistent syntax
+that can replace most use cases for the ``%`` formatting operator,
+``str.format()`` and ``string.Template``.
+
+F-strings can also perform better than alternatives; see
+`this tweet <https://twitter.com/raymondh/status/1205969258800275456>`_ for
+a simple example.

doc/data/messages/c/consider-using-f-string/good.py

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+menu = ('eggs', 'spam', 42.4)
+
+f_string_order = f"{menu[0]} and {menu[1]}: {menu[2]:0.2f} ¤"