docs: more release docs improvements

[kevinjqliu]

Rationale for this change

Following #3030, more docs improvements

• Add instruction to push the patch branch
• Add checks that env vars are set
• Upload to SVN without downloading all the artifacts first

Are these changes tested?

Yes, tested the svn import command locally

Are there any user-facing changes?

[geruh]

Thanks Kevin, this is a good addition to guard against running with unset variables.

I tested a few with examples like this and looks good.

> export VERSION=0.8.0
export RC=1
export VERSION_WITH_RC=${VERSION}rc${RC}
export GIT_TAG=pyiceberg-${VERSION_WITH_RC}

> echo svn import "svn-release-candidate-${VERSION}rc${RC}" \
  "https://dist.apache.org/repos/dist/dev/iceberg/pyiceberg-${VERSION_WITH_RC}" \
  -m "PyIceberg ${VERSION_WITH_RC}"

svn import svn-release-candidate-0.8.0rc1 https://dist.apache.org/repos/dist/dev/iceberg/pyiceberg-0.8.0rc1 -m PyIceberg 0.8.0rc1