Use furo-ys

[KrystalDelusion]

• [x] No more brand color overrides
• [x] Theme provides default Yosys logo and favicon
• [x] No more template override for right sidebar
• [x] No more custom css
• [x] Logo links to main docs
• [x] Search results includes a button to search all docs
• [ ] Nice looking search all docs button
• [x] Theme provides pygments lexers
  • [x] Yosys script
  • [x] Front end configs (e.g. .sby)
  • [x] Lexing for SBY style tags
• [ ] Theme provides common image helper classes
  • [ ] width-helper (is this common?)
  • [x] invert-helper