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>

site/assets/scss/_sidebar.scss

@@ -1,87 +1,53 @@
 .bd-sidebar {
-  @include media-breakpoint-down(md) {
-    margin: 0 ($bd-gutter-x * -1) 1rem;
-  }
-}
-
-.bd-links {
-  overflow: auto;
-  font-weight: 600;
-
-  @include media-breakpoint-up(md) {
+  @include media-breakpoint-up(lg) {
     position: sticky;
     top: 5rem;
     // Override collapse behaviors
     // stylelint-disable-next-line declaration-no-important
     display: block !important;
-    height: subtract(100vh, 7rem);
+    height: subtract(100vh, 6rem);
     // Prevent focus styles to be cut off:
     padding-left: .25rem;
     margin-left: -.25rem;
     overflow-y: auto;
   }
+}
 
-  > ul {
-    @include media-breakpoint-down(md) {
-      padding: 1.5rem .75rem;
-      background-color: $gray-100;
-      border-bottom: 1px solid $gray-200;
-    }
+.bd-links-nav {
+  @include media-breakpoint-down(lg) {
+    font-size: .875rem;
   }
 
-  a {
-    padding: .1875rem .5rem;
-    margin-top: .125rem;
-    margin-left: 1.25rem;
-    color: rgba($black, .65);
-    text-decoration: if($link-decoration == none, null, none);
+  @include media-breakpoint-between(xs, lg) {
+    column-count: 2;
+    column-gap: 1.5rem;
 
-    &:hover,
-    &:focus {
-      color: rgba($black, .85);
-      text-decoration: if($link-hover-decoration == underline, none, null);
-      background-color: rgba($bd-violet, .1);
-    }
-  }
-
-  .btn {
-    // Custom styles (as we don't have a completely neutral button style)
-    padding: .25rem .5rem;
-    font-weight: 600;
-    color: rgba($black, .65);
-    background-color: transparent;
-    border: 0;
-
-    &:hover,
-    &:focus {
-      color: rgba($black, .85);
-      background-color: rgba($bd-violet, .1);
+    .bd-links-group {
+      break-inside: avoid;
     }
 
-    &:focus {
-      box-shadow: 0 0 0 1px rgba($bd-violet, .7);
+    .bd-links-span-all {
+      column-span: all;
     }
-
-    // Add chevron if there's a submenu
-    &::before {
-      width: 1.25em;
-      line-height: 0; // Align in the middle
-      content: escape-svg($sidebar-collapse-icon);
-      @include transition(transform .35s ease);
-      transform-origin: .5em 50%;
-    }
-
-    &[aria-expanded="true"] {
-      color: rgba($black, .85);
-
-      &::before {
-        transform: rotate(90deg);
-      }
-    }
-  }
-
-  .active {
-    font-weight: 600;
-    color: rgba($black, .85);
+  }
+}
+
+.bd-links-link {
+  padding: .1875rem .5rem;
+  margin-top: .125rem;
+  margin-left: 1rem;
+  color: rgba($black, .65);
+  text-decoration: if($link-decoration == none, null, none);
+
+  &:hover,
+  &:focus,
+  &.active {
+    color: rgba($black, .85);
+    text-decoration: if($link-hover-decoration == underline, none, null);
+    background-color: rgba(var(--bd-violet-rgb), .1);
+  }
+
+  &.active {
+    font-weight: 600;
   }
 }