Document the different places where defaults are set

[arschles]

Is your feature request related to a problem? Please describe.

Discussion here has revealed that there are a lot of "layers" of defaults, and that is confusing.

Describe the solution you'd like

It would be helpful to have them documented, so that an Athens operator can understand where they should override defaults, depending on how they're deploying it.

For example, if someone is deploying Athens to Kubernetes via the helm chart, they should probably set defaults from the values.yaml file or from the helm command line.

Describe alternatives you've considered

N/A

Additional context

This discussion came from gomods/athens#1420 (comment)

[arschles]

I'll take this one 😄