[FEATURE REQUEST] - Creation of External Logger is undocumented

[jynolen]

Describe the solution you'd like
The documentation here (https://docs.saltstack.com/en/latest/ref/configuration/logging/handlers/index.html#external-logging-handlers) describe the list of external logger defined out of the box of salt
But this is undocumented that it is possible to create new loggers in extmod folder by putting
log_handlers subfolder

Additional context
Taking logstash as exemple external logging requires both __virtual__() , setup_handlers() functions to operate.
setup_handlers() return a valid object that respect logging.handler interface