chore(examples): removes nested docs, redirects, virtual fields, custom server, and hierarchy examples (#9112)

Removes examples that are now duplicative or unnecessary due to new
features in 3.0:

### Custom server

This one can be removed in favor of [Next.js
documentation](https://nextjs.org/docs/pages/building-your-application/configuring/custom-server)

### Hierarchy

The new `join` field can solve for many of the use cases for the
`hierarchy` example. Bi-directional relationships with the `join` field
should be preferred here.

### Nested Docs, Redirects

Our website template showcases how to use the `nested-docs` and
`redirects` plugins in-depth, with real-world examples.

### Virtual Fields

Virtual fields have gotten significantly easier and can now be defined
by specifying `virtual: true`. Not a big need for a full example any
longer.

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Co-authored-by: Jacob Fletcher <jacobsfletch@gmail.com>