Object Cache Overview Doc Update

[jazzsequence]

Re: Object Cache Overview

Priority: Medium

Issue Description:

Nowhere in our Object Cache documentation do we say what versions of Redis are available to users and how to change them. Our documentation in pantheon.yml tells you how to set/change the Redis version, but if I didn't know that pantheon.yml was where the Redis version was configured, I wouldn't know to look at that doc for the information.

Suggested Resolution

Somewhere in the Object Cache overview we should say something about the versions of Redis that are available and link to that section of the pantheon.yml documentation for guidance on how to configure it.

Relates to #8834 in that ideally we would have a page that listed all the various versions of all the things that exist on Pantheon (beyond just external libraries and in a format that's not expressly about pantheon.yml configuration).