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/search/index.ja.md

@@ -0,0 +1,13 @@
+---
+title: "検索"
+slug: "search"
+layout: "search"
+outputs:
+    - html
+    - json
+menu:
+    main:
+        weight: -60
+        params: 
+            icon: search
+---

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

@@ -0,0 +1,13 @@
+---
+title: "搜索"
+slug: "search"
+layout: "search"
+outputs:
+    - html
+    - json
+menu:
+    main:
+        weight: -60
+        params: 
+            icon: search
+---

demo/content/page/search/index.zh.md

@@ -0,0 +1,13 @@
+---
+title: "搜索"
+slug: "search"
+layout: "search"
+outputs:
+    - html
+    - json
+menu:
+    main:
+        weight: -60
+        params: 
+            icon: search
+---