Docs clean up: Merge and delete content from `routing/index.mdx` (#73859)
Please merge this PR first: https://github.com/vercel/next.js/pull/73858
Closes: https://linear.app/vercel/issue/DOC-4049/delete-routingindexmdx
The content for this page has already been moved to the `Getting
Started` section or `API Reference`.
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Co-authored-by: Rich Haines <hello@richardhaines.dev>