docs: add icons for extensions

docs/api/marks/bold.md

@@ -1,5 +1,6 @@
 ---
 description: Make your text bold and let it stand out.
+icon: bold
 ---
 
 # Bold

docs/api/marks/code.md

@@ -1,5 +1,6 @@
 ---
 description: Developers love to add some inline code to their texts.
+icon: code-view
 ---
 
 # Code

docs/api/marks/highlight.md

@@ -1,5 +1,6 @@
 ---
 description: Make it look nerdier with some colorful text highlights.
+icon: mark-pen-line
 ---
 
 # Highlight

docs/api/marks/italic.md

@@ -1,5 +1,6 @@
 ---
 description: Helps to emphasize your text, doesn’t bring you closer to Italy though.
+icon: italic
 ---
 
 # Italic

docs/api/marks/link.md

@@ -1,5 +1,6 @@
 ---
 description: Link it, link it good, link it real good (and don’t forget the href).
+icon: link
 ---
 
 # Link

docs/api/marks/strike.md

@@ -1,5 +1,6 @@
 ---
 description: Cut through the words you wrote, if you’re too afraid to delete it.
+icon: strikethrough
 ---
 
 # Strike

docs/api/marks/subscript.md

@@ -1,5 +1,6 @@
 ---
 description: Write slightly below the normal line, to show you’re unique.
+icon: subscript
 ---
 
 # Subscript

docs/api/marks/superscript.md

@@ -1,5 +1,6 @@
 ---
 description: Write slightly above the normal line, to show you’re just next level.
+icon: superscript
 ---
 
 # Superscript

docs/api/marks/text-style.md

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 ---
 description: Useless extension, just adds <span> tags (required by other extensions though).
+icon: text
 ---
 
 # TextStyle

docs/api/marks/underline.md

@@ -1,5 +1,6 @@
 ---
 description: Add a line below your text to make it look more … underlined.
+icon: underline
 ---
 
 # Underline