feat: Table of Contents (#232)

Currently, the new table of contents is disabled by default. I will change that after several versions.

-------------

* feat: Table of Content (WIP)

* fix: set scratch inside define block

* fix(toc): darkmode text color

* feat: unify page layout, and add config to disable toc

* feat: add scroll-behavior: smooth to html

* fix: use <ol> for TOC. And Disable TOC by default for now

* refactor: use css flexbox for article page with toc

enable toc for example site

* feat(i18n): add i18n entry for "Back" and "Table of Contents"

* style: remove unused `keep-sidebar` class

* doc: add table of contents as feature

assets/scss/partials/sidebar.scss

@@ -1,5 +1,4 @@
 .sidebar {
-    padding: 0 15px;
     &.sticky {
         @include respond(md) {
             position: sticky;
@@ -22,8 +21,8 @@
 
     @include respond(md) {
         width: auto;
-        margin-right: 1%;
-        padding: var(--main-top-padding) 15px;
+        padding-top: var(--main-top-padding);
+        padding-bottom: var(--main-top-padding);
         max-height: 100vh;
     }
 
@@ -46,7 +45,6 @@
     }
 
     @include respond(lg) {
-        margin-left: 1%;
         padding-top: var(--main-top-padding);
     }
 }
@@ -55,7 +53,7 @@
     z-index: 1;
     transition: box-shadow 0.5s ease;
 
-    padding: 15px 30px;
+    padding: 15px;
 
     @include respond(md) {
         padding: 0;