Debian installation guide updates
Debian installation guide updates

 * Explain Erlang version pinning so that unwanted
   OTP 20 upgrades can be avoided.

 * Warn IN ALL CAPS that Erlang/OTP 20 is unsupported and will
   lead to major issues if upgraded to (new installations are OK).

 * Mention "service rabbitmq-server" as a way to manage RabbitMQ service.

 * Formatting.

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website