move nodes and marks to a separate folder, add missing extensions

docs/src/docPages/api/nodes/heading.md

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+# Heading
+The Heading extension adds support for headings of different levels. Headings are rendered with `<h1>`, `<h2>`, `<h3>`, `<h4>`, `<h5>` or `<h6>` HTML tags. By default all six heading levels (or styles) are enabled, but you can pass an array to only allow a few levels. Check the usage example to see how this is done.
+
+Type <code>#&nbsp;</code> at the beginning of a new line and it will magically transform to a heading, same for <code>##&nbsp;</code>, <code>###&nbsp;</code>, <code>####&nbsp;</code>, <code>#####&nbsp;</code> and <code>######&nbsp;</code>.
+
+## Installation
+```bash
+# with npm
+npm install @tiptap/extension-heading
+
+# with Yarn
+yarn add @tiptap/extension-heading
+```
+
+## Settings
+| Option | Type   | Default            | Description                                   |
+| ------ | ------ | ------------------ | --------------------------------------------- |
+| class  | string | –                  | Add a custom class to the rendered HTML tag.  |
+| levels | Array  | [1, 2, 3, 4, 5, 6] | Specifies which heading levels are supported. |
+
+## Commands
+| Command | Options | Description             |
+| ------- | ------- | ----------------------- |
+| heading | level   | Creates a heading node. |
+
+## Keyboard shortcuts
+* Windows/Linux: `Control`&nbsp;`Alt`&nbsp;`1-6`
+* macOS: `Cmd`&nbsp;`Alt`&nbsp;`1-6`
+
+## Source code
+[packages/extension-heading/](https://github.com/ueberdosis/tiptap-next/blob/main/packages/extension-heading/)
+
+## Usage
+<demo name="Extensions/Heading" highlight="3-11,23,42-44" />