[Backport 10.4] Add information about configuring Redis cache (#2071)

* Add information about configuring Redis cache

Information and an example is added for
configuring Redis cache.

* Update Index.rst

- Remove warning box

Co-authored-by: Sybille Peters <sypets@gmx.de>
Co-authored-by: Tom Warwick <tom.warwick@typo3.org>

Documentation/ApiOverview/CachingFramework/FrontendsBackends/Index.rst

@@ -483,6 +483,73 @@ which must be available on the system.
    contains some important hints on how to speed up the system if it reaches bounds.
    A full documentation of available options is far beyond this documentation.
 
+
+.. _caching-backend-redis-example:
+
+Redis example
+-------------
+
+The Redis caching backend configuration is very similar to that of other
+backends, but there is one caveat.
+
+TYPO3 caches should be separated in case the same keys are used.
+This applies to the `pages` and `pagesection` caches.
+Both use "tagIdents:pageId_21566" for a page with an id of 21566.
+How you separate them is more of a system administrator decision. We provide 
+examples with several databases but this may not be the best option 
+in production where you might want to use multiple cores (which do not
+support databases). The separation has the additional advantage that
+caches can be flushed individually.
+
+If you have several of your own caches which each use unique keys (for example
+by using a different prefix for the cache identifier for each cache), you can
+store them in the same database, but it is good practice to separate the core
+caches.
+
+.. intentional quote!
+
+   In practical terms, Redis databases should be used to separate different keys
+   belonging to the same application (if needed), and not to use a single Redis
+   instance for multiple unrelated applications.
+
+   https://redis.io/commands/select/
+
+
+.. code-block:: php
+   :caption: public/typo3conf/AdditionalConfiguration.php
+
+   $redisHost = '127.0.0.1';
+   $redisPort = 6390;
+   $redisCaches = [
+       'pages' => [
+            'defaultLifetime' => 86400*7,
+            'compression' => true,
+       ],
+       'pagesection' => [
+            'defaultLifetime' => 86400*7,
+       ],
+       'hash' => [],
+       'rootline' => [],
+   ];
+   $redisDatabase = 0;
+   foreach ($redisCaches as $name => $values) {
+       $GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations'][$name]['backend']
+         = \TYPO3\CMS\Core\Cache\Backend\RedisBackend::class;
+       $GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations'][$name]['options'] = [
+           'database' => $redisDatabase++,
+           'hostname' => $redisHost,
+           'port' => $redisPort
+       ];
+       if (isset($values['defaultLifetime'])) {
+              $GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations'][$name]['options']['defaultLifetime']
+                  = $values['lifetime'];
+       }
+       if (isset($values['compression'])) {
+              $GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations'][$name]['options']['compression']
+                  = $values['compression'];
+       }
+   }
+
 .. _caching-backend-redis-options:
 
 Options