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/layouts/partials/footer.html

@@ -1,5 +1,5 @@
-<footer class="bd-footer py-5 mt-5 bg-light">
-  <div class="container py-5">
+<footer class="bd-footer py-4 py-md-5 mt-5 bg-light">
+  <div class="container py-4 py-md-5 px-4 px-md-3">
     <div class="row">
       <div class="col-lg-3 mb-3">
         <a class="d-inline-flex align-items-center mb-2 link-dark text-decoration-none" href="/" aria-label="Bootstrap">
@@ -20,6 +20,7 @@
           <li class="mb-2"><a href="/docs/{{ .Site.Params.docs_version }}/examples/">Examples</a></li>
           <li class="mb-2"><a href="{{ .Site.Params.themes }}">Themes</a></li>
           <li class="mb-2"><a href="{{ .Site.Params.blog }}">Blog</a></li>
+          <li class="mb-2"><a href="{{ .Site.Params.swag }}">Swag Store</a></li>
         </ul>
       </div>
       <div class="col-6 col-lg-2 mb-3">