add the character count extension

docs/src/docPages/api/extensions/character-count.md

@@ -0,0 +1,25 @@
+# CharacterCount
+[![Version](https://img.shields.io/npm/v/@tiptap/extension-character-count.svg?label=version)](https://www.npmjs.com/package/@tiptap/extension-character-count)
+[![Downloads](https://img.shields.io/npm/dm/@tiptap/extension-character-count.svg)](https://npmcharts.com/compare/@tiptap/extension-character-count?minimal=true)
+
+The `CharacterCount` extension limits the number of allowed character to a specific length. That’s it, that’s all.
+
+## Installation
+```bash
+# with npm
+npm install @tiptap/extension-character-count
+
+# with Yarn
+yarn add @tiptap/extension-character-count
+```
+
+## Settings
+| Option | Type     | Default | Description                                              |
+| ------ | -------- | ------- | -------------------------------------------------------- |
+| limit  | `Number` | `0`     | The maximum number of characters that should be allowed. |
+
+## Source code
+[packages/extension-character-count/](https://github.com/ueberdosis/tiptap-next/blob/main/packages/extension-character-count/)
+
+## Usage
+<demo name="Extensions/CharacterCount" highlight="" />