Document How to List Installed Extensions

[tmountain]

Improve documentation

Link

https://github.com/supabase/dbdev

Describe the problem

There's nothing in the docs showing how to list installed packages. I had to dig through the CLI source code to find out how to do this.

Describe the improvement

Update the docs to inform users that they can run this query.

select name, default_version from pgtle.available_extensions()

Update the docs to inform users that the CLI supports this as well (example using local dev).

dbdev list --connection 'postgresql://postgres:postgres@localhost:54322/postgres'

Additional context

Docs for basic package management should always include install, list, update, and removal of packages.