docs: Add missing server configuration env vars (#5676)

# Description
<!-- Please include a summary of the changes and the related issue.
Please also include relevant motivation and context. List any
dependencies that are required for this change. -->



**Type of change**
<!-- Please delete options that are not relevant. Remember to title the
PR according to the type of change -->

- Documentation update

**How Has This Been Tested**
<!-- Please add some reference about how your feature has been tested.
-->

**Checklist**
<!-- Please go over the list and make sure you've taken everything into
account -->

- I added relevant documentation
- I followed the style guidelines of this project
- I did a self-review of my code
- I made corresponding changes to the documentation
- I confirm My changes generate no new warnings
- I have added tests that prove my fix is effective or that my feature
works
- I have added relevant notes to the CHANGELOG.md file (See
https://keepachangelog.com/)

Author: frascuchon

argilla/docs/reference/argilla-server/configuration.md

@@ -84,6 +84,10 @@ The following environment variables are useful only when PostgreSQL is used:
 
 - `ARGILLA_ELASTICSEARCH_CA_PATH`: Path to CA cert for ES host. For example: `/full/path/to/root-ca.pem` (Optional)
 
+- `ARGILLA_ES_RECORDS_INDEX_SHARDS`: Default number of elasticsearch/opensearch shards for each search index. (Default: `1`).
+
+- `ARGILLA_ES_RECORDS_INDEX_REPLICAS`: Default number of elasticsearch/opensearch replicas for each search index. (Default: `0`).
+
 ### Redis
 
 Redis is used by Argilla to store information about jobs to be processed on background. The following environment variables are useful to config how Argilla connects to Redis: