Improve mobile usability of doc.qt.io

Google search console says that around 25% of the pages of doc.qt.io have issues with 'Mobile Usability':

These reasons include:

• Clickable elements too close together
• Text too small to read
• Content wider than screen
• Viewport not set

This not only means that the pages are not as usable on mobile devices (which might be OK for a reference documentation), but also most likely negatively affects our page rank in searches.