feat: better demo site (#1241)

* demo: add image-gallery post

* demo: change language weight

* demo: add multilanguage about page

* demo: change title and url

* demo: add shortcode documentation with multi-language support

* demo: improve markdown guide post

* demo: improve Math Typesetting post

* demo: rename Markdown Syntax Guide post

* demo: remove emoji support post

* demo: remove placeholder text post

* demo: remove rich-content post

* demo: rename image gallery page bundle

* demo: add multilingual `Documentation` category

* demo: Increase pagination pager size from 3 to 5.

* demo: add multilingual links, search, and archives pages

* demo: remove `ar` from demosite

demo/content/page/archives/index.zh-hant-tw.md

@@ -0,0 +1,11 @@
+---
+title: "歸檔"
+date: 2019-05-28
+layout: "archives"
+slug: "archives"
+menu:
+    main:
+        weight: -70
+        params: 
+            icon: archives
+---