[DOCS] Update getting-started.asciidoc (#29518)

Highlighted that you can change shard counts using `_shrink` and `_split`.

Author: markwalkom

docs/reference/getting-started.asciidoc

@@ -93,7 +93,8 @@ Replication is important for two primary reasons:
 
 
 To summarize, each index can be split into multiple shards. An index can also be replicated zero (meaning no replicas) or more times. Once replicated, each index will have primary shards (the original shards that were replicated from) and replica shards (the copies of the primary shards).
-The number of shards and replicas can be defined per index at the time the index is created. After the index is created, you may change the number of replicas dynamically anytime but you cannot change the number of shards after-the-fact.
+
+The number of shards and replicas can be defined per index at the time the index is created. After the index is created, you may also change the number of replicas dynamically anytime. You can change the number of shards for an existing index using the {ref}/indices-shrink-index.html[`_shrink`] and {ref}/indices-split-index.html[`_split`] APIs, however this is not a trivial task and pre-planning for the correct number of shards is the optimal approach.
 
 By default, each index in Elasticsearch is allocated one primary shard and one replica which means that if you have at least two nodes in your cluster, your index will have one primary shard and another replica shard (one complete replica) for a total of two shards per index.