Document the snapshotter's configuration options

[aochagavia]

The overview mentions that some aspects of the snapshotter are configurable through a toml file, but I couldn't found a description of the available configuration options. It would be nice to list them somewhere (e.g. how can I disable background fetching of whole layers)?

As a workaround, in the meantime, it would be useful to have at least a link to the code that handles options.

PS: I've been opening a bunch of documentation-related issues lately, based on things I stumble upon when getting started with the project. Regardless of the issues I've found, I just wanted to mention this project is being hugely useful to me, so thanks a lot!