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/_toc.scss

@@ -15,6 +15,7 @@
 
     ul {
       padding-left: 0;
+      margin-bottom: 0;
       list-style: none;
 
       ul {
@@ -40,3 +41,47 @@
     }
   }
 }
+
+.bd-toc-toggle {
+  display: flex;
+  align-items: center;
+
+  @include media-breakpoint-down(sm) {
+    justify-content: space-between;
+    width: 100%;
+  }
+
+  @include media-breakpoint-down(md) {
+    border: 1px solid $border-color;
+    @include border-radius(.4rem);
+
+    &:hover,
+    &:focus,
+    &:active,
+    &[aria-expanded="true"] {
+      color: var(--bd-violet);
+      background-color: $white;
+      border-color: var(--bd-violet);
+    }
+
+    &:focus,
+    &[aria-expanded="true"] {
+      box-shadow: 0 0 0 3px rgba(var(--bd-violet-rgb), .25);
+    }
+  }
+}
+
+.bd-toc-collapse {
+  @include media-breakpoint-down(md) {
+    nav {
+      padding: 1.25rem;
+      background-color: var(--bs-gray-100);
+      border: 1px solid $border-color;
+      @include border-radius(.25rem);
+    }
+  }
+
+  @include media-breakpoint-up(md) {
+    display: block !important; // stylelint-disable-line declaration-no-important
+  }
+}