Don't default to localhost in examples in the documentation

[b-abel]

The usage examples in Hono's documentation assume in many places (especially in the User guides) that the Hono installation is accessible at localhost, either explicitly by specifying it as the host, or as an implicit default (e.g. in the Mosquitto clients). There is no longer a supported, documented way to deploy Hono such that this assumption is true. The most accessible approach from a user perspective is either using the sandbox if experimenting from a client's side or deploying to a Kubernetes cluster with the Helm chart. It would simplify the learning experience if the examples would better fit the used setup.

Proposal: Always set the host explicitly in example commands. On the question of which hostname to document, I can think of two possibilities:

1. everywhere the sandbox
2. a variable + a short note (1x per page) that refers to a central place that explains how to retrieve the hostname (like in the Gettings started guide).