move functionalize out of experimental namespace (#85742)
Did a very quick sanity check - it looks like functorch docs don't get the nice preview link that pytofch-bot gives for normal pytorch docs, so I built locally and scanned `html/generated/functorch.functionalize.html`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85742
Approved by: https://github.com/zou3519
