Getting Started Docs: Clean up `docs/app` and `docs/pages` pages (#78151)
Closes: https://linear.app/vercel/issue/DOC-4498/clean-up-appdocs
- Went through the FAQ on the page and made sure the information is
found in other places in our docs.
- With the IA changes, we shouldn't need a FAQ that duplicates
information.
- We can have a dedicated App Router FAQ page in the future - if there's
a need for it.