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/grid.scss

@@ -92,7 +92,6 @@
 
 main.main {
     min-width: 0;
-    padding: 0 15px;
     max-width: 100%;
     flex-grow: 1;
     padding-top: var(--main-top-padding);
@@ -101,12 +100,10 @@ main.main {
 .main-container {
     min-height: 100vh;
     align-items: flex-start;
+    padding: 0 15px;
+    column-gap: var(--section-separation);
 
     @include respond(md) {
-        padding: 0 10px;
-    }
-
-    @include respond(lg) {
         padding: 0 20px;
     }
 }