More reference

[marcobambini]

• Add our extensions as a reference in the README and examples
• https://antonz.org/install-sqlite-extension/#download-manually
• https://github.com/nalgeon/sqlean/blob/main/docs/install.md
• https://github.com/asg017
• https://github.com/asg017/sqlite-ecosystem?tab=readme-ov-file
• https://sqlite-utils.datasette.io/en/stable/