Add pagination section to docs.

First draft, suggestions for improvements are welcome. The snippet should serve as the idiomatic guide on how to get all pages for a given resource (a common use case without an obvious solution, hence the docs can be very helpful).

As per https://github.com/google/go-github/commit/3c771890bd500dbd9c25e887af028c5af0eb11a8#commitcomment-6544800.