Docs: Add PPR page to getting started section (#79076)
Closes: https://linear.app/vercel/issue/DOC-4653/partial-prerendering
Related: https://linear.app/vercel/issue/DOC-4555/ppr-and-dio
Redirects: https://github.com/vercel/front/pull/45481
- Creates a placeholder page for PPR in Getting Started.
- Reviews and updates content
- Adds examples
