this has been overdue for a while. we also add the `shell` language to
the bundle (in addition to nix and bash) for ShellSession highlighting.
perhaps we should use the full, regular distribution instead of a custom
build, but eh.
- Rectifies diverging CSS by combining
nixos/nixpkgs docs CSS
- Moves our custom Highlight.js loader in to
the hljs package
- Switches the nixos docs to use SVG
callouts too