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/_ads.scss

@@ -14,7 +14,7 @@
   @include font-size(.8125rem);
   line-height: 1.4;
   text-align: left;
-  background-color: rgba(0, 0, 0, .05);
+  background-color: $gray-100;
 
   a {
     color: $gray-800;
@@ -22,8 +22,7 @@
   }
 
   @include media-breakpoint-up(sm) {
-    max-width: 330px;
-    @include border-radius(4px);
+    @include border-radius(.5rem);
   }
 }