Reorganize and update Getting Started documentation

[jillianlhale]

Overview

This PR reorganizes the Getting Started documentation to improve structure and user experience.

Changes Made

• File reorganization: Moved and renamed files for better logical flow
  • start/index.md → start/getting-started/What-is-curvenote.md
  • start/organize-content.md → start/getting-started/create-project.md
• Content updates: Enhanced documentation in multiple getting-started files
• Configuration: Updated myst.yml for improved site structure
• Cleanup: Removed outdated files (sites/create-a-site.md, start/organize-content.md)

Files Changed

Modified

• myst.yml - Site configuration updates
• start/getting-started/install-cli.md - Content improvements
• start/getting-started/create-article-cli.md - Enhanced instructions
• start/getting-started/publish-article.md - Updated guidance

Renamed

• start/index.md → start/getting-started/What-is-curvenote.md
• start/getting-started/organize-content.md → start/getting-started/create-project.md

Deleted

• sites/create-a-site.md - Outdated content
• start/organize-content.md - Moved to new location

Impact

• Better organized documentation structure
• Improved user onboarding experience
• Cleaner file hierarchy
• More intuitive navigation flow

[jillianlhale]

Ready for your review @rowanc1 @fwkoch :)