DOC-3197: mark export plugin as deprecated (#3700)

Author: michalnieruchalski-tiugo

modules/ROOT/pages/export.adoc

@@ -6,6 +6,8 @@
 :pluginname: Export
 :plugincode: export
 
+IMPORTANT: The {pluginname} plugin is now *deprecated* and will be permanently removed in the upcoming {productname} 9.0 release. To ensure continued access to similar functionality, consider using the link:https://www.tiny.cloud/docs/tinymce/latest/exportpdf/[Export to PDF^] Premium plugin as an alternative.
+
 include::partial$misc/requires_5_5v.adoc[]
 
 include::partial$misc/premiumplugin.adoc[]