Apollo Client 3.0 will provide a new getting started experience
that doesn't involve a separate package like `apollo-boost`.

Apollo Client 3.0 will include a new out of the box getting
started experience. That experience is still being worked out,
but since we're interested in trying to keep the docs somewhat
up to date as we work on AC 3.0 changes, we can't leave the
existing `apollo-boost` content in place (since boost is
being removed). This commit modifies the docs to remove all
references to boost, and replace all `ApolloClient` imports
with their equivalent coming from the new `@apollo/client`
package. We're still going to do a complete review/overhaul
of the docs as we finalize more parts of AC 3.0, but this
initial step will at least get boost out of the picture.

It's important to note that the docs changes made in this
commit are not yet usable, as `@apollo/client` hasn't been
published. While this might seem a bit "cart before the horse",
we need to get boost out of the picture before we can
properly restructure the Apollo Client repo, to prep for
`@apollo/client`.