support marix table marks

Author: jeana-redhat

contributing_to_docs/doc_guidelines.adoc

@@ -1591,6 +1591,11 @@ This renders as:
 > . Item 2
 > . Item 3
 
+=== Support matrix tables
+
+In tables that indicate version support, feature inclusion, or similar with a simple single-character indicator, use a capital letter "X" (`X`) centered in the cell.
+Other options, such as Unicode and emoji checkmarks, might lack screen reader support or require alt text.
+
 === Footnotes
 
 Avoid footnotes when possible.