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/categories/Documentation/_index.ja.md

@@ -0,0 +1,9 @@
+---
+title: ドキュメント
+description: "Stack テーマの使用方法に関する投稿"
+slug: "documentation"
+image: "hutomo-abrianto-l2jk-uxb1BY-unsplash.jpg"
+style:
+    background: "#2a9d8f"
+    color: "#fff"
+---

demo/content/categories/Documentation/_index.md

@@ -0,0 +1,9 @@
+---
+title: Documentation
+description: "Posts about how to use Stack theme"
+slug: "documentation"
+image: "hutomo-abrianto-l2jk-uxb1BY-unsplash.jpg"
+style:
+    background: "#2a9d8f"
+    color: "#fff"
+---

demo/content/categories/Documentation/_index.zh-hant-tw.md

@@ -0,0 +1,9 @@
+---
+title: 文檔
+description: "關於如何使用 Stack 主題的文章"
+slug: "documentation"
+image: "hutomo-abrianto-l2jk-uxb1BY-unsplash.jpg"
+style:
+    background: "#2a9d8f"
+    color: "#fff"
+---

demo/content/categories/Documentation/_index.zh.md

@@ -0,0 +1,9 @@
+---
+title: 文档
+description: "关于如何使用 Stack 主题的文章"
+slug: "documentation"
+image: "hutomo-abrianto-l2jk-uxb1BY-unsplash.jpg"
+style:
+    background: "#2a9d8f"
+    color: "#fff"
+---