docs: link manual Slack setup to Creating Custom Slack Apps section

Addresses CodeRabbit review comment on PR #2059 — gives readers a
direct pointer to the step-by-step manual setup section instead of
leaving them to figure it out.

https://claude.ai/code/session_019GQrc8wVq14if8f7F3pp4f

Author: claude

docs/configuration/sinks/slack.rst

@@ -27,7 +27,7 @@ Note: Robusta can only write messages and doesn't require read permissions.
 
 **Option 2: Manual Configuration**
 
-Generate a Slack API key on your own, then add it to your ``generated_values.yaml``:
+Generate a Slack API key on your own (see :ref:`Creating Custom Slack Apps` below), then add it to your ``generated_values.yaml``:
 
 .. code-block:: yaml