docs: note publish task is experimental and gated (#2635)

Adds a caution admonition to the Publish task guide indicating the
feature is experimental and gated, pointing users to support to request
access.

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Marcel Rebro <marcel.rebro@apify.com>

Author: 3 people

sources/platform/actors/publishing/publish-task.mdx

@@ -9,6 +9,12 @@ sidebar_position: 2
 
 This guide walks you through the publication process on the **Publication** tab and the steps to make a task public.
 
+:::caution Experimental feature
+
+Task publishing is an experimental feature and is not available to all users yet. To try it out, contact [Apify support](https://apify.com/contact).
+
+:::
+
 ## Prerequisites
 
 Before you publish a task, make sure you have: