Updates MDX docs (#56378)
When trying to update my own website to use App Router, I found the current Next.js Docs were missing specifics to get it working (most notably, no mention of `pageExtensions`). Coupled with community feedback, we decided to revisit and rework the entire page.
The page has been walked through and tested on brand new create-next-app projects for both App and Pages Routers to ensure completeness.
Closes [DX-2095](https://linear.app/vercel/issue/DX-2095/improve-nextjs-mdx-docs)
Rendered pages while running locally, in case they are more helpful in seeings the changes and different between routers.
### App Router
### Pages Router
