move all extensions to separate pages

docs/src/docPages/api/extensions/underline.md

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+# Underline
+Allows you to use the `<u>` HTML tag in the editor.
+
+#### Options
+*None*
+
+#### Commands
+| command | options | description |
+| ------ | ---- | ---------------- |
+| underline | none | Mark text as underlined. |
+
+#### Keybindings
+* Windows & Linux: `Control` + `U`
+* macOS: `Command` + `U`
+
+#### Example
+
+```markup
+<template>
+  <div>
+    <editor-menu-bar :editor="editor" v-slot="{ commands, isActive }">
+      <button type="button" :class="{ 'is-active': isActive.underline() }" @click="commands.underline">
+        Underline
+      </button>
+    </editor-menu-bar>
+
+    <editor-content :editor="editor" />
+  </div>
+</template>
+
+<script>
+import { Editor, EditorContent, EditorMenuBar } from 'tiptap'
+import { Underline } from 'tiptap-extensions'
+
+export default {
+  components: {
+    EditorMenuBar,
+    EditorContent,
+  },
+  data() {
+    return {
+      editor: new Editor({
+        extensions: [
+          new Underline(),
+        ],
+        content: `
+          <p><u>This is underlined.</u></p>
+          <p style="text-decoration: underline">This as well.</p>
+        `,
+      }),
+    }
+  },
+  beforeDestroy() {
+    this.editor.destroy()
+  }
+}
+</script>
+```