[7.2.beta] Add preface to books that were not updated for 7.2 beta

The content of the following books was not updated and they will not be presented on the 7.2 Beta documentation splash page.

• Migration Guide
• Patching and Upgrading Guide
• Using the Server Migration Tool
• Deploying Red Hat JBoss Enterprise Application Platform on Amazon EC2
• Using JBoss EAP in Microsoft Azure

Because other books may contain links to these books, we need to add a preface to say that they should not be used for 7.2 Beta.