docs: add contributing docs

[hoangndst]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

fix #13

2. What is the scope of this PR (e.g. component or file name):

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

This pull request adds a comprehensive CONTRIBUTING.md file to the kusion-backstage-plugin repository. It provides guidelines for contributing, coding standards, and the release process, along with instructions for setting up the development environment and creating changesets.

Changeset and Release Process:

• Explained when and how to create changesets for versioning and changelog updates, with links to relevant documentation.
• Documented the release process, including how changeset files trigger version bumps and package publishing to npm.

API Reports:

• Added instructions on generating and updating API Reports using API Extractor for plugins/packages, ensuring API documentation is up-to-date.

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

6. Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None