Add docs for using https between a singer and stacks-node

[wileyj]

ref: stacks-network/stacks-core#4741

we should add some docs on how a user may use https traffic between a signer and stacks-node.
we currently say to only run the two binaries on a single host, but there are some network topologies/strategies where encrypting the requests would be desired when running the binaries on separated instances.

an nginx proxy etc should be enough to address this (alongside letsencrypt or a self-signed cert).