Document configuration values and options

From photo/frontend#729. To copy/paste @jmathai in the original: 

"There's a handful of config options that can be set. These are typically in the following files.
- `src/configs/defaults.ini`
- `src/userdata/configs/{hostname}.ini`
- `src/html/assets/themes/{themename}/configs/settings.ini`

We need to document all of the standard ones and what their possible values are."
