feat: improve skip to content and add sr-only-focusable utility class
Our skip-to-content link should also be available for non-screen reader users who use keyboards.
This is commonly done on websites:
The CSS I used for "sr-only-focusable" is from Bootstrap 4: https://getbootstrap.com/docs/4.0/utilities/screenreaders/.
Bootstrap 5 offers completely different classes than sr-only and sr-only-focusable. I went with the Bootstrap 4 version as it is more compatible with Bootstrap 3. Once we migrate to BS5, we'll use their new classes.