Remove Internet Explorer from contents

[jmuheim]

As discussed in #274, we are updating our relevant combos of screen readers and browsers to the newest WebAIM survey 8, which allows us to remove IE completely from the supported combos.

This raises a lot of questions:

• What does this mean to the guide in general now?
• Can we modernise a lot of examples (e.g. use more ARIA)?
  • The complete "Widgets" section was coded with IE11 compatibility in mind. All of those examples are still valid, but some of them might benefit from an overhaul (e.g. using ARIA tabs instead of radio button tabs). Many suggestions could co-exist next to each other.
• When having multiple suggested solutions, should we prioritise them differently now (suggesting more modern ones before older, more robust ones)?
• General question: do we still need the compatibility info below each example?

This needs discussion.