Place footer below main content

[parlough]

Update the site's layout implementation so that the main content can scroll with the entire viewport rather than having a separate scroll view. This allows the footer to be placed after the main content rather than fixed at the bottom and better aligns the site's layout with dart.dev and docs.flutter.dev.

• Don't limit the body to a height of 100%, so that the main content can be scrolled as part of the primary/full viewport. This is what allows the footer to be after the content.
• Make header sticky so they always remain visible at the top of the viewport.
• Make sidebars sticky, but below the header, so that they remain visible when scrolling the main content.
• Remove custom webkit scrollbar styles as they cause part of the header's chrome to go missing and are platform dependent.
  • Add color-scheme: <light or dark> to the theme styles so the browser provides properly themed scrollbars.
  • Removing these styles also resolves a TODO relating to the mobile version of the sidebars.

Resolves #4013

[parlough]

I tested locally with Flutter, Dart, and the testing package. I recommend staging at least testing-package yourself to get a feel of the change:

dart run tool/task.dart serve testing-package

[parlough]

I staged the flutter docs at: https://parlough-api-dart-dev.firebaseapp.com/doc/