docs(soroban): clarify support status and reorganize docs

[mohamedbasuony]

Summary

This PR improves and reorganizes the Soroban target documentation.

• expand docs/targets/soroban.rst into a clearer landing page
• add dedicated pages for the support matrix, language compatibility, example coverage, and Rust SDK differences
• update the main README.md to point readers to the Soroban documentation and current support status
• add a maintenance note to make support expectations explicit

Notes

• docs only
• no compiler or runtime behavior changes
• intended to make the current Soroban status easier to understand for contributors and users