Documentation: don't debug-print overly long data URLs

When QWebEnginePage::setHtml() is called, the URL's string
representation becomes "data:text/html;charset=UTF-8,PAGE", where PAGE
stands for the entire HTML contents. If kdevplatform.documentation.debug
is true, KDevelop prints a very long line each time a CMake or Man
documentation page is loaded. Such long lines are not particularly
useful but can slow the application down.

Print a general message in place of a URL with the "data" scheme.