Document how to install older version of Percona Toolkit

With the new repositories, it is possible to install older versions of our packages. We need to document this process so users understand how to do it.

apt: https://linoxide.com/linux-command/install-specific-version-package-apt-get/

yum: see top answer https://unix.stackexchange.com/questions/151689/how-can-i-instruct-yum-to-install-a-specific-version-of-package-x

Note: It has also been observed that the meta packages have trouble with this feature. In order to install an older version. The version must be specified on all packages and dependencies for that package (that are hosted in our repository).

Note: Currently, Debian multi-version support in our original repository location, older products that are only available through the original repository location should not include this documentation.