Add ECH in preview tags to the EDOT Cloud Forwarder page (#492)

Making sure that the ECH preview availability on the [EDOT Cloud Forwarder](https://www.elastic.co/docs/reference/opentelemetry/edot-cloud-forwarder) page is visible.

This change ensure continuity with the updates in the [Compare Elastic Cloud Hosted and Serverless](https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings) page that compares various capabilities between ECH and Serverless. 

This change relates to #491 and elastic/docs-content#1198 (comment)

Author: yetanothertw

docs/reference/edot-cloud-forwarder/index.md

@@ -4,6 +4,8 @@ description: Introduction to the EDOT Cloud Forwarder, the Elastic Distribution
 applies_to:
   serverless:
     observability: preview
+  deployment:
+    ess: preview
 products:
   - id: cloud-serverless
   - id: observability