Document how HMS federation works

[flyrain]

It's great to have this feature in Polaris. We will need to document the usage. cc @poojanilangekar @dennishuo

[poojanilangekar]

Hi @flyrain

I did add it as a README here: https://github.com/apache/polaris/blob/main/extensions/federation/hive/README.md

The rest of it works just as regular federation. Would you like me to add to some other document?

[flyrain]

@poojanilangekar, thanks for sharing it. For any users who want to use federation, it would be nice to provide a page with a step-by-step instructions to show how it works, we could create a new page here https://polaris.apache.org/in-dev/unreleased/. I guess it's not only for Hive catalog, but also for Hadoop catalog and Rest catalog.