MINOR: Add a command to make site-docs and Javadocs updates easier on version release

[clolov]

The Kafka website is hosted in a separate repository than the Kafka code. The files versioned in the kafka-site repository are generated as part of the version release process and then copied over. Today, this is a manual process detailed in https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=34840886#ReleaseProcess-Websiteupdateprocess

This PR aims to simplify the manual steps by adding a sub-command to the release.py script. The command uses the javadocs and *-site-docs.tgz in the .release_work_dir and copies them over to the kafka-site repository. There is a rudimentary dry-run sub-sub-command which can be used for path-checking.

The addition to the script makes the assumption that the kafka-site repository is on the same level as the kafka repository on a user's machine. In the suggestion for subsequent commands it makes the further assumption that the branch used in the kafka-site repository is asf-site.

There are many things which we can improve about the overall script and this addition in particular, but it is a start 😊.

I have tested this manually as part of the 4.0.1 release process and cross-checked with the manual changes I did in https://github.com/apache/kafka-site/pull/718 and https://github.com/apache/kafka-site/pull/719

[clolov]

Heya @mimaison, @showuon, @mjsax and @lianetm! I am proposing this improvement to the release process because it took me an embarrassing 😅 amount of time to figure it out as part of the 4.0.1 release. Let me know your thoughts!

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