This PR implements a restyling of the documentation page to match the look and feel of the new site.

Specifically, the restyling consists of:

• Using the new HTEP logo
• Matching the font family with HTEP site
• Increasing spacing between elements
• Redesigning topbar so:
  • It matches the new HTEP site look and feel
  • It adapts to small screens
• Hiding the TOC in small screens (it is already available in the top bar), so acual contents appear above the fold.

NB: There is another branch, called restyle, with the same changes, but the file ext/req.py has been adapted to work with Python 3.