Create a Lein job that prepares a new release

[bbatsov]

While deploying releases is now automated there are few prep tasks that need to be done every time:

• update the changelog (to add the release date and version)
• update references to the newest version in the README and the docs
• update the docs version in antora.yml

I was thinking we can have some simple lein task for this, unless someone has better ideas.