Improve Diffusion hosting setup instructions somewhat?

Summary:
Ref T10866. Fixes T10386. This attempts to make it a little more plausible to follow these directions:

• Use simpler language in general.
• Remove language suggesting that HTTP requires no additional configuration.
• Suggest using a load balancer or an ugly port number instead of swapping SSH to a different port.
• Be more granular about sudo setup.
• Organize better?

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10386, T10866

Differential Revision: https://secure.phabricator.com/D15796