Improving 'Day 2' Documentation

[bjflanne]

Based on some recurrent feedback from developer community, enhance documentation to cover topics of interest after initial set up of NATS. Examples include:

• Sizing NATS (single node and Clusters)
• Upgrading NATS (especially when clustered in an "always" on scenario)
• Troubleshooting NATS (what to look for in the logs, what to expect with debug or trace output, etc)

[ColinSullivan1]

Part of this PR is also described in #265