docs: add postgres example for sql catalog

[jdockerty]

Which issue does this PR close?

No specific issue, although I can raise one for this if required.

What changes are included in this PR?

This expands the documentation for the sql catalog implementation to include a postgres specific example.

Perhaps the usage was obvious enough from the current "generic" example, but I found that the example showing the SQL server URI as a HTTP server a little confusing. I've left this here as I think it is still useful, but also provided a more postgres tailored one in order to show "real database" usage, rather than simply filling in the values.

In any case, I don't think more documentation can hurt 😄

Are these changes tested?

N/A

[blackmwk]

Thanks @jdockerty for this pr!

[github-actions]

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[jdockerty]

@blackmwk I missed that this had a CI flake.

I've updated the PR and everything is passing now, are you fine with merging this as-is now?

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 1 week if no further activity occurs. If you think that’s incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the dev@iceberg.apache.org list. Thank you for your contributions.