Update lambda documentation and add auth docs

I was about to add my first lambda to my shared instance using this tutorial when I realized slash-graphql login wasn’t working and then I saw this.

If that blog post is no longer valid, why not update/delete it?

Also, the DGraph API Docs mention a link (https://dgraph.io/docs/cloud-api/authentication/) that points to nowhere. It’d be very helpful if the Docs for that part and for pushing lambdas could be updated.