Improve documentation navigation and organization

[asafashirov]

This PR improves the organization and navigation of our documentation files:

• Creates CLAUDE.md with essential commands and guidelines for Claude Code AI
• Consolidates UX-STANDARDS.md into STYLE-GUIDE.md for comprehensive guidance
• Enhances CONTRIBUTING.md with better navigation and organization
• Adds cross-references between all documentation files

These changes make our documentation more discoverable and easier to navigate while preserving all valuable technical content.

[pulumi-bot]

Your site preview for commit d242560c is ready! :tada:

http://www-testing-pulumi-docs-origin-pr-14876-d242560c.s3-website.us-west-2.amazonaws.com.