[release][docs] Update documents for the release of version 0.5.0

[forfudan]

This pull request updates documentation and examples to reflect the new v0.5.0 release of DeciMojo, improves clarity for both English and Chinese readers, and adds comprehensive usage examples for all major types. The changes ensure consistency with the latest features, update installation and compatibility instructions, and provide quick start guides for BigInt and Decimal128.

Documentation and Usage Examples:

• Added detailed example files: docs/examples_on_bigint.mojo and docs/examples_on_decimal128.mojo to demonstrate the construction, arithmetic, conversions, and advanced features of BigInt and Decimal128.
• Expanded and clarified the Chinese README (docs/readme_zht.md) with a full table of contents, improved type descriptions, installation instructions, and comprehensive quick start guides for BigInt, BigDecimal, and Decimal128.
• Updated the English README.md to clarify installation steps, version compatibility, and usage of type aliases, and to improve example explanations.

Release and Versioning Updates:

• Updated the changelog (docs/changelog.md) to reflect the new release date and summarize the new features and improvements in v0.5.0, including enhanced math functions, performance optimizations, and compatibility with Mojo v25.5.
• Updated package version requirements in pixi.toml and documentation tables to require Mojo v25.5 for DeciMojo v0.5.0, ensuring users install compatible versions.

Citation and Metadata:

• Updated citation and software metadata in the Chinese README to match the new version and scope of the library.

Other Improvements:

• Improved and clarified footnotes and technical explanations in the Chinese README, including details about internal representations and division semantics.

These changes ensure that documentation, examples, and metadata are up-to-date and provide clear guidance for both English and Chinese users.