[DOCS] Makes shards optional in Create pack API (#166639)

## Summary

* Resolves elastic/security-docs#3822.  

* Updates the `shards` object in Create pack API to optional for 8.10.1
and 8.11.0 onwards, per #166178

* Related to changes made in
#166363.

docs/api/osquery-manager/packs/create.asciidoc

@@ -33,7 +33,7 @@ experimental[] Create packs.
 
 `policy_ids`:: (Optional, array) A list of agents policy IDs.
 
-`shards`:: (Required, object) An object with shard configuration for policies included in the pack. For each policy, set the shard configuration to a percentage (1–100) of target hosts.
+`shards`:: (Optional, object) An object with shard configuration for policies included in the pack. For each policy, set the shard configuration to a percentage (1–100) of target hosts.
 
 `queries`:: (Required, object) An object of queries.