Create documentation for supporting multi database

[weifanwu]

Wrote the documentation to support multiple database connections (PostgreSQL, MySQL, SQLite) with DuckDB extensions.

TODO:

• (Implement feature that supports other databases don't have DuckDB extensions yet such as BigQuery, StarRocks or SnowFlakes)

Issue: #399

[jheer]

Thanks! Please see the various comments. Once addressed, this should be ready to merge.

In addition, I think this documentation should reside under /api/core/ instead of /api/duckdb/ (as the latter is specific to Mosaic's Node.js DuckDB server package).

[jheer]

Thanks @weifanwu!