Capitalize the word Markdown throughout the documentation

Closes #2435

content/en/quick-reference/emojis.md

@@ -1,6 +1,6 @@
 ---
 title: Emojis
-description: Include emoji shortcodes in your markdown or templates.
+description: Include emoji shortcodes in your Markdown or templates.
 categories: [quick reference]
 keywords: [emoji]
 menu:
@@ -11,13 +11,13 @@ weight: 20
 toc: true
 ---
 
-Configure Hugo to enable emoji processing in markdown:
+Configure Hugo to enable emoji processing in Markdown:
 
 {{< code-toggle file=hugo >}}
 enableEmoji = true
 {{< /code-toggle >}}
 
-With emoji processing enabled, this markdown:
+With emoji processing enabled, this Markdown:
 
 ```md
 Hello! :wave: