Overhaul site and convert to Hugo

Jekyll posed repeated administrative headaches. This commit recreates
the site from scratch using Hugo as the static site generator and Congo
as the theme.

The most significant changes in appearance are:

• URLs are "pretty" (ending in / rather than .html)
• The former sidebar, which duplicated the former top menu, is gone
• The site supports dark mode (automatically following the local operating system preference, or manually toggled with a footer icon)

The most significant changes in content are:

• The top menu is now "Projects", "Community", "Help", "About", and site search
  • "Projects" is based on the former "components" page
  • "Community" collects community resources that were scattered on the sidebar and former "help" page
  • "Help" is a simplified section based on the former "Documentation", "FAQ", "Help", and "Developers" pages
  • "About" has project history
• Outdated information has been removed, including the entire "quickstart" section and most images
• Text has been greatly simplified

README.md has instructions for working with the new site.