Adapt guide to changing popularity of operating systems & screenreaders

[manuelmeister]

In the new Web Accessibility Practitioners survey, we see that macOS (and therefore voiceover) continues to gain popularity, while JAWS is dropping off sharply. This should probably be reflected in the guide.

• https://www.accessibility-developer-guide.com/knowledge/screen-readers/relevant-combinations/
• https://www.accessibility-developer-guide.com/setup/screen-readers/voiceover-ios/
• https://www.accessibility-developer-guide.com/setup/windows/why/

[NicSR]

@manuelmeister : WebAIM's screen reader user survey is now updated with the new statistics. Is this enough for you?
@christophdubach : Should we add additional info about the popularity?