docs: modernize rclone.org site design

- replace Bootstrap/jQuery with purpose-built CSS and JS
- remove backend icons from navbar and content pages
- replace remaining FontAwesome icons with inline SVGs, remove FontAwesome
- modernize CSS styling for menus, typography, cards, tables, and code blocks
- add copy-to-clipboard buttons on code blocks using SVG icon
- move TOC to left sidebar with responsive overlay drawer
- add sticky header, top scrollbar and first column for wide tables
- add left/right arrow buttons to scrollable tables
- hide homepage logo on mobile
- make wide menus with filter for Commands and Storage Systems
- add dark mode support based on browser preference
- fix CSS/JS cache busting to use build time

docs/layouts/partials/tier.html

@@ -25,7 +25,7 @@
 {{ else if .name }}
   {{/*  Backend Lookup Usage */}}
   {{ $backendName := .name | lower }}
-  {{ $data := index site.Data.backends $backendName }}
+  {{ $data := index hugo.Data.backends $backendName }}
   {{ if $data }}
     {{ $currentTier = $data.tier }}
   {{ end }}