Improve release page instructions

[CTTY]

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

[geruh]

I think the iceberg-python release scripts are a good starting point!

https://github.com/apache/iceberg-python/blob/main/mkdocs/docs/how-to-release.md