Copy config.dev.toml to the docs page

[arschles]

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

We get good questions about config directives in Athens. We don't have all the configs documented on docs.gomods.io, and it would be hard to keep the docs site up to date with the config file.

Describe the solution you'd like

The config file itself is well commented, so I'd like it to show up on the docs page.

Describe alternatives you've considered

We could manually write it out, but then we'd have to keep docs up to date with config.

Additional context
This is a specific proposal for how to achieve #1502