Top-level library documentation

DoD:
`bottlerocket_settings_sdk::lib` has a docstring containing:
* Context for the library's existence
* A quickstart guide
* References to common (anticipated) usage patterns
* Any gotchas that users should be aware of

Start at the crate's `bottlerocket_settings_sdk::lib` module and review its current documentation. Add context for the library, a quickstart, anticipated usage patterns, and user-facing gotchas; done means the module docstring contains all four items.

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