Improve Docs

[bh2smith]

Making this better:

https://docs.rs/duners/latest/duners/

[cursor]

PR Summary

Low Risk
Primarily documentation and public API ergonomics (re-exports and error trait impl) with no changes to request/response logic; low runtime risk but minor breaking risk for users relying on old module paths.

Overview
Improves public documentation by rewriting the README and adding extensive crate/module rustdoc, including clearer quick-start examples, auth/parameter guidance, and deserialization helpers.

Enhances API ergonomics by re-exporting common types at the crate root (DuneClient, DuneRequestError, Parameter, ExecutionStatus, GetResultResponse) and by making DuneRequestError implement Display + std::error::Error for better propagation/logging.

Mostly refines docs/comments on DuneClient, Parameter, parse_utils, and response types, and updates examples to use the new root-level imports and no_run doctests.

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