Use an ico for the favicon in root so that it works in all browsers.

Apparently IE 10 still ignores png files for favicons.
Include a 32x32 ico too for high dpi screens.

Notes: I'm not aware of another way to make jekyll copy favicon.ico to the root out dir if I place it in docs-assets/ico that's why I put it in the repository root. If anyone knows an alternative please let me know.