Upgrade from Smarty 3 to Smarty 4 to be PHP 8.1 compatible

Remove all Smarty4 dedicated tests, all are done in the same test file
like before

www/lib/smarty-4.1.0/docs/programmers/caching.md

@@ -0,0 +1,24 @@
+Caching
+=======
+
+Caching is used to speed up a call to [`display()`](./api-functions/api-display.md) or
+[`fetch()`](./api-functions/api-fetch.md) by saving its output to a file. If a cached
+version of the call is available, that is displayed instead of
+regenerating the output. Caching can speed things up tremendously,
+especially templates with longer computation times. Since the output of
+[`display()`](./api-functions/api-display.md) or [`fetch()`](./api-functions/api-fetch.md) is cached, one
+cache file could conceivably be made up of several template files,
+config files, etc.
+
+Since templates are dynamic, it is important to be careful what you are
+caching and for how long. For instance, if you are displaying the front
+page of your website that does not change its content very often, it
+might work well to cache this page for an hour or more. On the other
+hand, if you are displaying a page with a timetable containing new
+information by the minute, it would not make sense to cache this page.
+
+## Table of contents
+- [Setting Up Caching](./caching/caching-setting-up.md)
+- [Multiple Caches Per Page](./caching/caching-multiple-caches.md)
+- [Controlling Cacheability of Output](./caching/caching-groups.md)
+- [Custom Cache Implementation](./caching/caching-custom.md)