refactor: rewrite grid system & style changes (#231)

* refactor(WIP): add new SCSS mixin `respond`

Theme's stylesheet mixes the use of max-width and min-width for responsive design, and it's horrible.

* refactor: update variable.scss to use @mixin respond

* refactor(grid): adjust sidebar max-width

* refactor(partials/article.scss): use `@include respond`

* refactor(layout/article.scss): delete unused SCSS and migrate to `@include respond`

* Adjust left sidebar max-width on xl screen

* fix: right sidebar not sticky

* refactor(breakpoint): simplify `@mixin respond`

* refactor(breakpoints): improve warning message

* fix(breakpoint): return only the requested breakpoint

* fix: add missing padding to .article-details

assets/scss/breakpoints.scss

@@ -1,4 +1,17 @@
-$on-phone: 812px;
-$on-tablet: 1024px;
-$on-desktop: 1519px;
-$on-desktop-large: 1920px;
+$breakpoints: (
+    sm: 640px,
+    md: 768px,
+    lg: 1024px,
+    xl: 1280px,
+    2xl: 1536px,
+);
+
+@mixin respond($breakpoint) {
+    @if not map-has-key($breakpoints, $breakpoint) {
+        @warn "'#{$breakpoint}' is not a valid breakpoint";
+    } @else {
+        @media (min-width: map-get($breakpoints, $breakpoint)) {
+            @content;
+        }
+    }
+}