Document list of breaking changes for penta

[mcoker]

Placeholder for working on a list of breaking changes for penta, similar to this table we created for v5 - https://www.patternfly.org/get-started/upgrade/release-notes

The majority of this will likely be the changes from the old global variable system to tokens, and there may not be a 1:1 list of "v5 global var --pf-v5-some-var changed to to v6 token --pf-t-some-var". So we should discuss whether we need to try and create a v5->v6 map of vars/tokens, or if we can just point folks to the token documentation as a reference for how to update old global vars to tokens.