feat(Style): update responsive design of landing page

fix overflow issues in small screens, hide full navbar on mobile and replace for an off-canvas version

src/assets/style/_mixins.scss

@@ -0,0 +1,17 @@
+@use './variables' as *;
+
+@mixin responsive {
+  @each $size-name, $size in $screen-sizes {
+    @media screen and (min-width: $size) {
+      @content ($size-name);
+    }
+  }
+}
+
+@mixin responsive-steps($from, $to) {
+  @include responsive using ($size-name) {
+    @for $index from $from through $to {
+      @content ($size-name, $index);
+    }
+  }
+}