Document config.toml basics

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

Our [configuration documentation](https://docs.gomods.io/configuration/) has a lot of specific information, but very little general overview on how to configure Athens.

**Describe the solution you'd like**

We should have documentation explaining the `config.toml` file, environment variables, and even the configuration that you can do via the helm chart.

**Describe alternatives you've considered**

N/A

**Additional context**

This should be probably be solved at the same time as https://github.com/gomods/athens-charts/issues/29