Upgrades and doc fixes
* Upgrade to Gradle `5.2.1`
* Upgrade Kotlin, Asciidoc, Sonarqube, Checkstyle, AssertJ
* Store Docs in the separate `/html` and `/pdf` dirs in the final zip,
as it was in the previous versions
* Fix Docs to render (or not) version properly: we don't need a version
on front HTML page just after author names in the same line.
So, introduce a simple CSS in the `docinfo.html` to hide that `revnumber`.
It looks OK, though, in PDF: rendered on the separate line
* Move `What's New?` content to the `appendix.adoc` and start section for
the current version
* Fix Checkstyle for suspicion failure about nested annotations