Docs: Improve `README.md` & `CONTRIBUTING.md`

[nick-y-ito]

Overview

This PR improves README.md & CONTRIBUTING.md.

Changes

• Polished the README.md and CONTRIBUTING.md
• Split the content of CONTRIBUTING.md into multiple sections

Review points

• Clear and easy to understand?
• Please provide any ideas for improvements.

Screen Captures

References

• I have partly referenced this repo to improve our docs https://github.com/radix-ui/primitives

Notes

There are still some remaining issues that need to be addressed for the documentation update.

• 277

• 279

• 280

Assignee Checklist:

• [x] The naming convention of the PR title is correct (See the comment at the top of this template)
• [x] The base branch is correct (See: Types of Branches)
• [x] The branch name follows the Branch Naming Conventions
• [x] The correct assignees and reviewers have been designated for this PR
• [x] The coding style follows the Coding Style Guide
• [x] All the related issues are associated with this PR
• [x] All criteria in the associated issues are met (please tick the checkboxes)
• [x] My changes do not generate new warnings or errors (especially in the console)

Reviewer Checklist:

• [ ] Double-check the "Assignee Checklist"
• [ ] The code follows the generic best practices and our Coding Style Guide
• [ ] The code is well-commented and easy to understand
• [ ] The UI changes accurately reflect the provided design

[nick-y-ito]

@kanta1207 Thank you for your review :)