Link documentation formats

* Add a link from the HTML to the PDF and vice-versa.
* Update doc link text
* I missed that we have three formats: single-page HTML, multiple HTML pages, and PDF.
This commit links the single-page HTML and PDF to the other two formats.

Author: Jay Bryant

src/reference/asciidoc/index-header.adoc

@@ -4,6 +4,12 @@ Mark Fisher; Marius Bogoevici; Iwein Fuld; Jonas Partner; Oleg Zhurakousky; Gary
 
 ifdef::backend-html5[]
 *{revnumber}*
+
+NOTE: This documentation is also available as https://docs.spring.io/spring-integration/docs/current/reference/html/index.html[multiple (faster to load) HTML pages] and as https://docs.spring.io/spring-integration/docs/current/reference/pdf/spring-integration-reference.pdf[PDF].
+endif::[]
+
+ifdef::backend-pdf[]
+NOTE: This documentation is also available as https://docs.spring.io/spring-integration/docs/current/reference/html/index.html[multiple (faster to load) HTML pages] and as a single https://docs.spring.io/spring-integration/docs/current/reference/html/index-single.html[(more easily searchable) HTML file].
 endif::[]
 
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