docs: update contribution guidelines

[benjaminach]

Summary

This PR updates CONTRIBUTING.md with clearer documentation contribution guidelines.

Changes include:

• Adding writing style guidelines for clear international English
• Clarifying that documentation pages use kramdown Markdown
• Documenting how to use Jekyll includes for reusable content
• Updating redirect guidance to append new entries at the end of the 301 section
• Clarifying that existing redirects should not be modified during normal page moves