Reorganize DuckDB and PostgreSQL Extension Documentation

[semihsalihoglu-uw]

We currently have a lot of duplication of the documentation across DuckDB and PostgreSQL extensions. Specifically:

• How to INSTALL/LOAD
• Clear attached database schema cache
• USE statement

are almost identical. The overall running examples in both documentations are also almost identical. The only part where they meaningfully differ are in how you enter the records into the PostgreSQL or DuckDB databases.

Ideally we should restructure these pages into a single page where parts of the documentation could be expandable or separate tabs, where the commands for DuckDB, Postgres, and later MySQL etc. could be shown.

[prrao87]

Done:
https://docs.kuzudb.com/extensions/rdbms/