docs: update draftMode guide (#94155)
Reframes the Draft Mode guide so the headline is "every Next.js cache
layer is bypassed for the request," not "switch to dynamic rendering."
The previous guide led with an `isEnabled` fork in `getData()`, which
implied users have to branch their fetch code to make caching step
aside. They don't.
Changes:
- Intro lists the bypassed layers (`fetch` cache, `'use cache'`,
`unstable_cache`, ISR response cache).
- Step 3 fetches from a single endpoint with no `isEnabled` branch.
- New Step 4 shows a preview banner + disable handler, which is where
`isEnabled` is actually useful.
- The "Cache Components" example uses `isEnabled` for a UI indicator
instead of branching the fetch URL.
- Two-URL fork demoted to a final section for CMSs that genuinely need
it.
