Update the demo page (index.html)

[jasonwebb]

Demo page viewable at: https://accessible360.github.io/accessible-slick/

This should have a similar format to the original package's doc page so its nice and familiar, but without feeling like a rip-off.

• [X] Set up Github Pages for this repo
• [x] Update index.html to match the content being served on the original Slick Slider repo.
  • Looks like the actual page being served is not the index.html in the original repo. The rendered DOM will need to be scrapped and cleaned up, then put into the index.html in this repo.
• [x] Move index.html to docs/ so supporting files can be added without cluttering up the root folder.
• [x] Update the color scheme to differentiate it from the original doc page.
• [X] Fix the heading structure (there is a skipped heading above Single Items)
• [x] Hide <hr> tags from screen readers to cut down on noise.
• [X] Remove the heading markup around the "and a whole lot more..." pseudo-heading, since it isn't semantically a heading.
• [x] Add HTML5 landmark elements (<header>, <nav>, and <main>).
• [x] Add a "skip to main content" link.
• [x] Implement better focus styles throughout.
• [X] Add a "Sponsored by" section at the top.