docs: make it clear that you can run the gateway inside the same process

[tmc]

There seems to be some community confusion around possible deployment paths. We should have better docs that show how you can embed the gateway into your grpc server.

[novabyte]

@tmc can you show an example or link to somewhere in the code on how to embed the gateway?

[johanbrandhorst]

https://coreos.com/blog/grpc-protobufs-swagger.html is always my go-to when I need to show someone using them in parallel.