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

i18n/en.yaml

@@ -17,8 +17,15 @@ list:
         other: Subsections
 
 article:
+    back:
+        other: Back
+
+    tableOfContents:
+        other: Table of contents
+
     relatedContents:
         other: Related contents
+
     lastUpdatedOn:
         other: Last updated on
 
@@ -32,8 +39,10 @@ widget:
     archives:
         title:
             other: Archives
+
         more:
             other: More
+
     tagCloud:
         title:
             other: Tags
@@ -41,13 +50,16 @@ widget:
 search:
     title:
         other: Search
+
     placeholder:
         other: Type something...
+
     resultTitle:
         other: "#PAGES_COUNT pages (#TIME_SECONDS seconds)"
 
 footer:
     builtWith:
         other: Built with {{ .Generator }}
+
     designedBy:
         other: Theme {{ .Theme }} designed by {{ .DesignedBy }}