Link watermark blog to doc (#7036)

## Summary
Cross-link new [watermark
blog](https://www.elastic.co/search-labs/blog/elasticsearch-disk-watermark-troubleshooting)
from [watermark
doc](https://www.elastic.co/docs/troubleshoot/elasticsearch/fix-watermark-errors).

## Generative AI disclosure
1. Did you use a generative AI (GenAI) tool to assist in creating this
contribution?
- [ ] Yes  
- [X] No

---------

Co-authored-by: shainaraskas <58563081+shainaraskas@users.noreply.github.com>

Author: stefnestor

troubleshoot/elasticsearch/fix-watermark-errors.md

@@ -51,6 +51,8 @@ The following are some common setup issues leading to watermark errors:
 * A high number of replicas can quickly multiply storage requirements, as each replica consumes the same disk space as the primary shard. Refer to [Index settings](elasticsearch://reference/elasticsearch/index-settings/index-modules.md) for details.
 * Oversized shards can make disk usage spikes more likely and slow down recovery and rebalancing. Refer to [Size your shards](/deploy-manage/production-guidance/optimize-performance/size-shards.md#shard-size-recommendation) for guidance.
 
+For more causes and remediation guidance, refer to the Elastic blog post [Why your Elasticsearch cluster is hitting disk watermarks](https://www.elastic.co/search-labs/blog/elasticsearch-disk-watermark-troubleshooting).
+
 ## Monitor disk usage [fix-watermark-errors-monitor]
 
 :::{include} /deploy-manage/_snippets/autoops-callout-with-ech.md