Improve release page instructions

> A couple of TODO to improve the release page:
> - Format commands properly (and clearly)
> - change `svn co https://dist.apache.org/repos/dist/dev/iceberg/ /tmp/iceberg-dist-dev` to not download the entire svn locally
> - add variable guard (assert that variables are set before the command)
> - fix quoting when copy/pasting command
> - fix variables in command (make sure they are consistent)
> - format email
>

_Originally posted by @kevinjqliu in [#2213](https://github.com/apache/iceberg-rust/issues/2213#issuecomment-4034747900)_

Start from the release page instructions referenced by the issue and review each listed command and the email text. Done means commands are clearly formatted and copyable, the checkout avoids downloading the entire SVN tree, variables are guarded and consistent, quoting works, and the email is properly formatted.

Written by the indexing model from the issue text.