chore(github): remove /examples from contribution guidelines, remove examples issue template (#77050)
## Why?
Start the prep-work for removing/moving [Next.js examples](https://github.com/vercel/next.js/tree/canary/examples).
Today, a lot of these examples already have their own dedicated location in their respective GitHub orgs.
These examples are also now present in other locations.
- https://github.com/nextjs
- https://v0.dev/chat/community
- https://vercel.com/templates/next.js