Document running multiple services on single vm

[Automaat]

Description

We should improve our docs for this case, and point users to using net namespaces and cgorups

[jijiechen]

FYI. some newly introduced features ("application probe proxy", and "Envoy readiness reporter") listen a local port on the host, so we need to make sure they use different ports when run multiple instances on a single VM.

The configuration keys are:

• KUMA_READINESS_PORT configures on which port the "Envoy readiness reporter" listens, defaults to 9902.
• KUMA_APPLICATION_PROBE_PROXY_PORT configures on which port the "application probes proxy" listens, defaults to 9001. It can be disabled by set to 0 when the feature is not used.