[DOCS] Adding a known issue for 8.13 (elastic#179471)

Adding a known issue to the 8.13 release notes. Canvas workpad is
displaying a toast notification stating image rendering failed.

![Screenshot 2024-03-26 at 17 23
26](https://github.com/elastic/kibana/assets/61687663/b29948e1-271b-4bd5-8353-cee52b19bc3f)

Relates to: elastic#179457

---------

Co-authored-by: Nick Peihl <nickpeihl@gmail.com>

docs/CHANGELOG.asciidoc

@@ -66,6 +66,19 @@ Review important information about the {kib} 8.x releases.
 
 For information about the {kib} 8.13.0 release, review the following information.
 
+[float]
+[[known-issues-8.13.0]]
+=== Known issues
+
+[discrete]
+[[known-179457]]
+.In Canvas, an empty text element incorrectly triggers a toast notification
+[%collapsible]
+====
+*Details* +
+In Canvas, an empty text element incorrectly triggers a "Markdown content is required in [readOnly] mode" toast notification. For more information, refer to ({kibana-pull}179457[#179457]).
+====
+
 [float]
 [[breaking-changes-8.13.0]]
 === Breaking changes