Update header and footer to match new site design

[cjyabraham]

This repeats work from this PR to adapt to the new docs theme, bringing it in to alignment with the new PyTorch redesign.

Header now looks like this:

And footer looks like this:

A few notes:

• I tweaked the color of $pytorch-red to match what's on the live site. Note this color was chosen so that it meets WCAG AA accessibility contrast requirements. The original color does not.
• The footer social icons don't exactly match the live site, however, I think they're close enough and look a bit sharper than the live site.

[svekars]

Thanks so much, Chris!