Improve task flow through https://docs.chef.io/server_security.html#chef-server-credentials-management

[davidwrede]

• Make the docs clearer on where to set the insecure_addon_compat setting
• Improve task flow for encrypting and managin secrets/passwords

[brewn]

I think the first bullet point here is fixed with https://github.com/chef/chef-web-docs/pull/1115. Will take a look at the second one this week.