DEVELOPER-1104 Added in JDG 6.4 Beta to the available downloads - Release notes currently not working

Also, to update the 'Green button', don't we need to change the default download?
Add the following below here:

    url: https://access.redhat.com/documentation/en-US/Red_Hat_JBoss_Data_Grid/6.4/html/6.4.0_Beta_Release_Notes/index.html