Add links to alternate documentation formats (#1348)

* Add links to alternate documentation formats

Link the HTML to the PDF and vice-versa.

* Update src/reference/asciidoc/index.adoc

Co-authored-by: Gary Russell <[email protected]>

Co-authored-by: Gary Russell <[email protected]>

Author: Jay Bryant

src/reference/asciidoc/index.adoc

@@ -12,6 +12,12 @@ Mark Pollack; Mark Fisher; Oleg Zhurakousky; Dave Syer; Gary Russell; Gunnar Hil
 
 ifdef::backend-html5[]
 *{project-version}*
+
+NOTE: This documentation is also available as https://docs.spring.io/spring-amqp/docs/current/reference/pdf/spring-amqp-reference.pdf[PDF].
+endif::[]
+
+ifdef::backend-pdf[]
+NOTE: This documentation is also available as https://docs.spring.io/spring-amqp/docs/current/reference/html/index.html[HTML].
 endif::[]
 
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