add placeholder extension

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

@@ -0,0 +1,29 @@
+# Placeholder
+[![Version](https://img.shields.io/npm/v/@tiptap/extension-placeholder.svg?label=version)](https://www.npmjs.com/package/@tiptap/extension-placeholder)
+[![Downloads](https://img.shields.io/npm/dm/@tiptap/extension-placeholder.svg)](https://npmcharts.com/compare/@tiptap/extension-placeholder?minimal=true)
+
+This extension provides placeholder support.
+
+## Installation
+```bash
+# with npm
+npm install @tiptap/extension-placeholder
+
+# with Yarn
+yarn add @tiptap/extension-placeholder
+```
+
+## Settings
+| Option               | Type                | Default               | Description                                                 |
+| -------------------- | ------------------- | --------------------- | ----------------------------------------------------------- |
+| emptyEditorClass     | `String`            | `'is-editor-empty'`   | The added CSS class if the editor is empty.                 |
+| emptyNodeClass       | `String`            | `'is-empty'`          | The added CSS class if the node is empty.                   |
+| placeholder          | `String | Function` | `'Write something …'` | The placeholder text added as `data-placeholder` attribute. |
+| showOnlyWhenEditable | `Boolean`           | `true`                | Show decorations only when editor is editable.              |
+| showOnlyCurrent      | `Boolean`           | `true`                | Show decorations only in currently selected node.           |
+
+## Source code
+[packages/extension-placeholder/](https://github.com/ueberdosis/tiptap-next/blob/main/packages/extension-placeholder/)
+
+## Usage
+<demo name="Extensions/Placeholder" />