[repo][docs] Reorganize the repo + update the readme file

[forfudan]

This pull request includes significant updates to the README.md file and several changes to benchmark files, mainly focusing on improving documentation and code organization.

Documentation Enhancements:

• README.md: Expanded the overview and added detailed descriptions of the Decimal type and future roadmap items. Enhanced the installation instructions and added a comprehensive quick-start guide.
• Status Section: Updated the status of the project phases, detailing the completion of the "Make it Work" phase, progress in the "Make it Right" phase, and significant advancements in the "Make it Fast" phase. Added a new "Future Extensions" section.
• Footnotes: Added footnotes to explain the fixed precision and arbitrary precision types, providing context and comparisons to similar implementations in other languages.

Codebase organization:

• Dedicated folder for Decimal type: Added a dedicated decimal sub-package for the Decimal type. Also added dedicated folders for the tests and benches.