Docs framework - update unified/felt theme name in theme switcher

[mcoker]

We need to update the following

• Change the class it applies from .pf-v6-theme-redhat to .pf-v6-theme-felt

• Update the "Theme" toggle group text from "Unified" to "Felt"

There is other logic/localStorage and stuff that references both "redhat" and "unified" that should be updated at some point, too, but the bullets above are the most important.

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Jira Issue: PF-4055