docs: redesign doc/index and update admin/install

Created by: bobheadxi

part of #22425 (closed) and #22427 (closed) - improves docsite paths to deployment docs (also IMO this is a bit overdue 😛 )

• Redesign docs homepage
  • Use logo and copy market-y intro text from https://about.sourcegraph.com/
  • "Quick start guide" is now "Quick install". IMO this is a bit clearer on the intent that the Docker deployment is just for the sake of something that's quick to install, and not really to use.
  • Remove a lot of the quick install guide that's duplicated from the docker install guide
  • More prominent note about docker-not-for-production
  • Sourcegraph Cloud and deployment methods are now much more prominently featured
• Update admin/install
  • Feature self-hosted more prominently, reorder the options and add some formatting
  • Updated tips
• A bit of README update

Preview

sg run docsite