Adds .jsx to doc guidelines

Author: opayne1

contributing_to_docs/doc_guidelines.adoc

@@ -1036,6 +1036,7 @@ syntax highlighting. For example:
 ** `[source,yaml]`
 ** `[source,go]`
 ** `[source,javascript]`
+** `[source,jsx]`
 
 * Do not use more than one command per code block. For example, the following must
 be split up into three separate code blocks: