Document recommended usage

[silvanmelchior]

So far, the documentation only describes how ConfZ can be used, but not how it is recommended to use.

A separate section in the config could go into more details about:

• Complete nested config for whole project vs separate module-level config
• How deep down in your layers should config still be used, when should specific values be passed down instead
• When to use dataclasses instead of ConfZ classes
• When to depend on typing.Protocol instead of ConfZ class
• When to use local configs without singleton pattern
• How to structure your project using ConfZ (e.g. file structure)