Add the documentation pages and apply the XWiki conventions

{{Add the documentation of the extension for www.xwiki.org (user, administrator and developer pages, with their screenshots) and bring the code in line with the XWiki conventions: }}
{{multi-line Javadoc comments, }}
{{jakarta.inject, }}
{{the root cause message in the logs, }}
{{-ntp in the Maven commands, }}
ordered functional tests with the browser details kept in the page objects, and the xwiki- prefix for the JavaScript module names.