docs: comprehensive documentation improvements across Learn, Build, Operate, and Reference sections

[Dark-Brain07]

Summary

This PR contains 7 meaningful documentation improvements:

1. Clarinet Quickstart - Exit instructions, common mistakes, next steps
2. Stacks.js Overview - Package descriptions table, quick example
3. Learn README - Prerequisites for learners
4. Operate README - Troubleshooting tips, community support, health checks
5. Build README - Fixed broken Clarinet links
6. Networks.md - API endpoints table, rate limit warning
7. Main README - Contribution guidelines, quick links, Get Help section

Testing

• All markdown syntax is valid
• All internal links point to existing files
• No breaking changes to existing documentation