Overhaul developer documentation

[mlafeldt]

Catch-all issue for improving README.md and CONTRIBUTING.md

• Clarify use of duckdb-rs as a database client vs for DuckDB extensions
  • panic: DuckDB API not initialized or DuckDB feature omitted #536
• Document C API thread safety
  • Document C API thread safety #453
  • QUESTION: is Connection thread safe?  #201
• How to load built extensions
  • Better docs for examples/hello-ext #181
• More useful examples
  • Improve docs #106
• Cross compilation
  • Cross compilation error #239

...