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/_callouts.scss

@@ -6,36 +6,30 @@
   padding: 1.25rem;
   margin-top: 1.25rem;
   margin-bottom: 1.25rem;
-  border: 1px solid $gray-200;
-  border-left-width: .25rem;
-  @include border-radius();
+  background-color: var(--bd-callout-bg, var(--bs-gray-100));
+  border-left: .25rem solid var(--bd-callout-border, var(--bs-gray-300));
 
   h4 {
     margin-bottom: .25rem;
   }
 
-  p:last-child {
+  > :last-child {
     margin-bottom: 0;
   }
 
-  code {
-    @include border-radius();
-  }
-
   + .bd-callout {
     margin-top: -.25rem;
   }
+
+  .highlight {
+    background-color: rgba($black, .05);
+  }
 }
 
 // Variations
-.bd-callout-info {
-  border-left-color: $bd-info;
-}
-
-.bd-callout-warning {
-  border-left-color: $bd-warning;
-}
-
-.bd-callout-danger {
-  border-left-color: $bd-danger;
+@each $variant in $bd-callout-variants {
+  .bd-callout-#{$variant} {
+    --bd-callout-bg: rgba(var(--bs-#{$variant}-rgb), .075);
+    --bd-callout-border: rgba(var(--bs-#{$variant}-rgb), .5);
+  }
 }