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/_layout.scss

@@ -1,4 +1,7 @@
 .bd-layout {
+  padding-right: $bd-gutter-x;
+  padding-left: $bd-gutter-x;
+
   @include media-breakpoint-up(md) {
     display: grid;
     gap: $grid-gutter-width;