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/config/_default/params.toml

@@ -6,7 +6,7 @@ favicon        = ""
     since = 2020
 
 [sidebar]
-    emoji    = "🍥"
+    emoji    = "✏️"
     subtitle = "Lorem ipsum dolor sit amet, consectetur adipiscing elit."
     [sidebar.avatar]
         enabled = true