Fix broken links to README (#14093)

changelog.d/14093.doc

+Fix links to jemalloc documentation, which were broken in #13491.

docker/README.md

@@ -241,4 +241,4 @@ healthcheck:
 
 Jemalloc is embedded in the image and will be used instead of the default allocator.
 You can read about jemalloc by reading the Synapse
-[README](https://github.com/matrix-org/synapse/blob/HEAD/README.rst#help-synapse-is-slow-and-eats-all-my-ram-cpu).
+[Admin FAQ](https://matrix-org.github.io/synapse/latest/usage/administration/admin_faq.html#help-synapse-is-slow-and-eats-all-my-ramcpu).

docs/usage/configuration/config_documentation.md

@@ -1139,7 +1139,7 @@ number of entries that can be stored.
 
 * `cache_autotuning` and its sub-options `max_cache_memory_usage`, `target_cache_memory_usage`, and
    `min_cache_ttl` work in conjunction with each other to maintain a balance between cache memory
-   usage and cache entry availability. You must be using [jemalloc](https://github.com/matrix-org/synapse#help-synapse-is-slow-and-eats-all-my-ramcpu)
+   usage and cache entry availability. You must be using [jemalloc](../administration/admin_faq.md#help-synapse-is-slow-and-eats-all-my-ramcpu)
    to utilize this option, and all three of the options must be specified for this feature to work. This option
    defaults to off, enable it by providing values for the sub-options listed below. Please note that the feature will not work
    and may cause unstable behavior (such as excessive emptying of caches or exceptions) if all of the values are not provided.