Add descriptive text to download statistics

Reported on https://www.linkedin.com/posts/activity-7417169461817794560-FTQL

Text could read "Statistics are generated from binary downloads only. They do not include downloads from the repositories, or any other channel outside of mariadb.org."

Please confirm that this is indeed the case - there was talk of including distro stats in some way, but I don't think this got anywhere.