Update Qt docs

* Added Qt 5.12 docs (latest LTS release)
* Use a stable URL for Qt 5.11 docs

Author: tronical

lib/docs/scrapers/qt.rb

@@ -103,9 +103,14 @@ class Qt < UrlScraper
       Licensed under the GNU Free Documentation License, Version 1.3.
     HTML
 
+    version '5.12' do
+      self.release = '5.12'
+      self.base_url = 'https://doc.qt.io/qt-5.12/'
+    end
+
     version '5.11' do
       self.release = '5.11'
-      self.base_url = 'https://doc.qt.io/qt-5/'
+      self.base_url = 'https://doc.qt.io/qt-5.11/'
     end
 
     version '5.9' do