Jetstream Documentation

[ColinSullivan1]

Integrate documentation at https://github.com/nats-io/jetstream here. Enhance and add as necessary.

See #178 (The Jetstream readme is being translated into documentation)

This includes:

• [ ] General Description and Applicable Use Cases
• [x] Concepts
• [x] Getting Started Guide - Simple setup based on our recommendations.
• [x] Server documentation
• [x] Basic parameters/configuration options
• [ ] Security
• [ ] Developing clients with Jetstream
• [x] Monitoring
• [x] Administration and usage (CLI)
• [ ] Securing Jetstream

Section could be under: https://github.com/nats-io/nats.docs/tree/master/jetstream.

Of course, add additional documentation as necessary.

/CC @gcolliso