fix(admin): add explicit labels to caching configuration headings

APCu, Redis, and Memcached h2 headings on the memory caching page were
generating non-descriptive IDs (id1, id2, ...) instead of the expected
anchors (apcu, redis, memcached). Adding explicit RST reference labels
ensures stable, predictable fragment URLs.

Also fix a double-space typo in the existing _install_redis_label
directive that made it malformed RST.

Fixes #6878

Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

Author: skjnldsv

admin_manual/configuration_server/caching_configuration.rst

@@ -111,7 +111,6 @@ APCu is faster for local caching than Redis. If you have enough memory, use
 APCu for Memory Caching and Redis for File Locking. If you are low on memory,
 use Redis for both.
 
-
 APCu
 ----
 
@@ -341,8 +340,7 @@ prevent session corruption when using Redis as your session handler: ::
 More information on configuration of phpredis session handler can be found on the
 `PhpRedis GitHub page <https://github.com/phpredis/phpredis>`_
 
-..  _install_redis_label:
-
+.. _install_redis_label:
 
 Memcached
 ---------