Docs offcanvas navbar (#34487)

* Redesign docs navbar to use offcanvas, tweak overall padding while I'm here

* Match code snippets to new gutter width

* Replace value with docs-specific variable

Opted not to do a CSS variable here since on .bd-content it would be inherited into our examples

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

site/assets/scss/_sidebar.scss

@@ -1,6 +1,6 @@
 .bd-sidebar {
   @include media-breakpoint-down(md) {
-    margin: 0 -.75rem 1rem;
+    margin: 0 ($bd-gutter-x * -1) 1rem;
   }
 }