v5.2.0 design refresh, plus responsive offcanvas classes (#35736)

* Add responsive offcanvas classes

- Updates navbar-expand classes to de-dupe some styles—these shouldn't interfere now.
- Adds some JS to the offcanvas component to help with responsiveness

Co-Authored-By: GeoSot <geo.sotis@gmail.com>

* Redesign homepage, docs, and examples

Homepage:

- New Bootstrap purple navbar
- Redesigned masthead
- Rewrote and redesigned homepage content
- Replace Copy text with icons like Bootstrap Icons site across all ClipboardJS instances
- Fixed padding issues in site footer
- Match homepage button styles to examples page, use gap instead of tons of responsive margin utils

Docs:

- New navbar, no more subnav. Migrated search and version picker into the main navbar and refreshed the design of it all, including the responsive toggles.
- New sidebar navigation is always expanded, and now features Bootstrap Icons alongside section headings
- Sidebar navigation autoscrolls to active link for better usability
- Subnav and navbar padding issues ironed out
- Enhanced the version picker in anticipation of v5.2: we can now link right to the same page in the previous version.
- Redesign callouts to add more color to our pages
- Collapse table of contents on mobile
- Cleanup and redesign button styles with CSS variables
- Update design for subnav version dropdown
- Update highlight and example to be full-width until md
- Improve the Added In badges
- Turn the ToC into a well on mobile
- Redesign code snippets to better house two action buttons

Examples:

- Redesign Examples page layout
- Add new example for responsive offcanvases in navbars

* Convert offcanvas to CSS vars

* Feat: add resize handler to Offcanvas.js.

If we could use as default the `.offcanvas` class without modifiers, we then, could add a simplified selector
The selector itself, ignores the .offcanvas class as it doesn't have any responsive behavior
The `aria-modal` addon is to protect us, selection backdrop elements

* Separate examples code, Add some selectors, fix stackblitz btn

Co-authored-by: GeoSot <geo.sotis@gmail.com>

scss/_navbar.scss

@@ -222,41 +222,31 @@
           display: none;
         }
 
-        .offcanvas-header {
-          display: none;
-        }
-
         .offcanvas {
-          position: inherit;
-          bottom: 0;
+          // stylelint-disable declaration-no-important
+          position: static;
           z-index: auto;
           flex-grow: 1;
-          visibility: visible !important; // stylelint-disable-line declaration-no-important
-          background-color: transparent;
-          border-right: 0;
-          border-left: 0;
+          width: auto !important;
+          height: auto !important;
+          visibility: visible !important;
+          background-color: transparent !important;
+          border: 0 !important;
+          transform: none !important;
           @include box-shadow(none);
           @include transition(none);
-          transform: none;
-        }
-        .offcanvas-top,
-        .offcanvas-bottom {
-          height: auto;
-          border-top: 0;
-          border-bottom: 0;
-        }
+          // stylelint-enable declaration-no-important
 
-        .offcanvas-body {
-          display: flex;
-          flex-grow: 0;
-          padding: 0;
-          overflow-y: visible;
-        }
+          .offcanvas-header {
+            display: none;
+          }
 
-        // Reset `background-color` in case `.bg-*` classes are used in offcanvas
-        .offcanvas,
-        .offcanvas-body {
-          background-color: transparent !important; // stylelint-disable-line declaration-no-important
+          .offcanvas-body {
+            display: flex;
+            flex-grow: 0;
+            padding: 0;
+            overflow-y: visible;
+          }
         }
       }
     }