FLUID-5469: Removing the mention of FSS

Removed FSS from docs and updated the prefs framework related documentation.
I think I agree: A single folder containing all migration docs, broken down by version. I don't necessarily think it should start with "tutorial-" though, and it's not really migrating "to Infusion."

Also, this might be the time to mention: Should we be starting on "migrating to 2.0" docs explaining how to remove FSS, in case anyone's using it? At the very least, any API requirements that used to require FSS files will need to have instructions on what to remove.