docs: Add comprehensive Gonfig derive macro documentation

[0xvasanth]

Summary

• Add comprehensive rustdoc documentation for the Gonfig derive macro
• Simplify derive macro code generation
• Fix clippy warnings in core library files

Changes

Documentation (gonfig_derive/src/lib.rs)

Added 153 lines of comprehensive documentation including:

• Generated Methods: Overview of from_gonfig(), from_gonfig_with_builder(), and gonfig_builder()
• Container Attributes: Detailed docs for env_prefix, allow_cli, allow_config
• Field Attributes: Complete coverage of env_name, cli_name, default, skip/skip_gonfig
• Configuration Priority: Clear explanation of how sources are merged
• Complete Example: Real-world usage demonstrating all features

Code Simplification

• Extract shared logic: Created reusable setup_env_cli code block to eliminate duplication between from_gonfig_with_builder() and gonfig_builder() methods
• Simplify config file loading: Replaced verbose if-else chain with clean loop over ["config.toml", "config.yaml", "config.json"]
• Clean up variable usage: Removed unused allow_env variable (environment is always enabled)
• Streamline environment key generation: Simplified logic with cleaner pattern matching
• Remove redundant comments: Cleaned up unnecessary inline comments

Code Quality

Fixed clippy::uninlined_format_args warnings in core library:

• src/builder.rs
• src/cli.rs
• src/config.rs

Benefits

1. Improved Developer Experience: Comprehensive documentation helps users understand all available options
2. Reduced Code Duplication: Shared environment/CLI setup logic now in one place (~40 lines eliminated)
3. Better Maintainability: Future changes only need to be made once
4. Cleaner Code Flow: Simplified config file loading is easier to understand
5. Modern Rust Style: All format strings follow inline variable conventions

Note on CI

This PR has clippy warnings in example files. These examples need the fixes from PR #15 to be merged first. The core library changes pass all checks.

Test Plan

• Documentation builds successfully
• Core library clippy passes with -D warnings
• All tests pass
• No functional changes to derive macro behavior
• Code simplification preserves exact same functionality

[Copilot]

Pull Request Overview

Add comprehensive rustdoc for the Gonfig derive macro, simplify derive code generation by extracting shared setup logic, and fix clippy inline format warnings in core files.

• Add detailed rustdoc for container/field attributes, method behavior, and examples
• Refactor derive macro generation: extract shared environment/CLI setup and simplify config-file discovery
• Update formatting to satisfy clippy::uninlined_format_args in core modules

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/config.rs	Switch to inline formatting in error messages to satisfy clippy and modern Rust style.
src/cli.rs	Use inline formatting for serialization error message.
src/builder.rs	Use inline formatting for deserialization error message.
gonfig_derive/src/lib.rs	Add comprehensive rustdoc and refactor macro generation (shared env/CLI setup, simpler config-file loop).

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[Copilot]

Documentation for allow_config should clarify the selection semantics and order. The implementation loads only the first existing file in this order: config.toml, then config.yaml, then config.json. Please update the docs to state 'the first existing file among [config.toml, config.yaml, config.json] is loaded (in that order)'.

Suggested change

	/// Automatically loads from `config.{toml,yaml,json}` in current directory.
	/// Loads the first existing file among [`config.toml`, `config.yaml`, `config.json`] in the current directory (in that order).

[Copilot]

Avoid unwrap() in proc-macros to provide helpful compile-time diagnostics for unsupported forms (e.g., tuple/unit structs). Instead, return a syn::Error with a clear message, e.g., return syn::Error::new_spanned(&f, \"Gonfig requires named fields\").to_compile_error().into(); or collect these errors and abort! accordingly.