add a demo and tests to the horizontal rule extension

docs/src/demos/Extensions/HorizontalRule/index.spec.js

@@ -0,0 +1,40 @@
+context('/api/extensions/horizontal-rule', () => {
+  beforeEach(() => {
+    cy.visit('/api/extensions/horizontal-rule')
+
+    cy.get('.ProseMirror').window().then(window => {
+      const { editor } = window
+      editor.setContent('<p>Example Text</p>')
+    })
+  })
+
+  describe('horizontal-rule', () => {
+    it('the button should add a horizontal rule', () => {
+      cy.get('.ProseMirror hr').should('not.exist')
+      cy.get('.demo__preview button:first').click({ force: true })
+      cy.get('.ProseMirror hr').should('exist')
+    })
+
+    it('the default markdown shortcut should add a horizontal rule', () => {
+      cy.get('.ProseMirror').window().then(window => {
+        const { editor } = window
+        editor.clearContent()
+
+        cy.get('.ProseMirror hr').should('not.exist')
+        cy.get('.ProseMirror').type('---', {force: true})
+        cy.get('.ProseMirror hr').should('exist')
+      })
+    })
+
+    it('the alternative markdown shortcut should add a horizontal rule', () => {
+      cy.get('.ProseMirror').window().then(window => {
+        const { editor } = window
+        editor.clearContent()
+
+        cy.get('.ProseMirror hr').should('not.exist')
+        cy.get('.ProseMirror').type('___ ', {force: true})
+        cy.get('.ProseMirror hr').should('exist')
+      })
+    })
+  })
+})

docs/src/demos/Extensions/HorizontalRule/index.vue

@@ -0,0 +1,54 @@
+<template>
+  <div v-if="editor">
+    <button @click="editor.focus().horizontalRule()">
+      horizontalRule
+    </button>
+
+    <editor-content :editor="editor" />
+  </div>
+</template>
+
+<script>
+import { Editor } from '@tiptap/core'
+import { EditorContent } from '@tiptap/vue'
+import Document from '@tiptap/extension-document'
+import Paragraph from '@tiptap/extension-paragraph'
+import Text from '@tiptap/extension-text'
+import HorizontalRule from '@tiptap/extension-horizontal-rule'
+
+export default {
+  components: {
+    EditorContent,
+  },
+
+  data() {
+    return {
+      editor: null,
+    }
+  },
+
+  mounted() {
+    this.editor = new Editor({
+      extensions: [
+        Document(),
+        Paragraph(),
+        Text(),
+        HorizontalRule(),
+      ],
+      content: `
+        <p>This is a paragraph.</p>
+        <hr>
+        <p>And this is another paragraph.</p>
+        <hr>
+        <p>But between those paragraphs are horizontal rules.</p>
+      `,
+    })
+
+    window.editor = this.editor
+  },
+
+  beforeDestroy() {
+    this.editor.destroy()
+  }
+}
+</script>

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

@@ -1,7 +1,7 @@
 # Blockquote
 The Blockquote extension enables you to use the `<blockquote>` HTML tag in the editor. This is great – you might have guessed – to use quotes in the editor.
 
-Type `> ` on the beginning of a new line and it will be magically transformed to a blockquote.
+Type `> ` at the beginning of a new line and it will be magically transformed to a blockquote.
 
 ## Options
 | Option | Type   | Default | Description                                  |

docs/src/docPages/api/extensions/horizontal-rule.md

@@ -1,5 +1,7 @@
 # HorizontalRule
-Enables you to use the `<hr>` HTML tag in the editor.
+Use this extension to render a `<hr>` HTML tag. If you pass `<hr>` in the editor’s initial content, it’ll be rendered accordingly.
+
+Type `---` (three dashes) or `___ ` (three underscores and a space) at the beginning of a new line and it will be magically transformed to a horizontal rule.
 
 ## Options
 | Option | Type   | Default | Description                                  |
@@ -9,11 +11,17 @@ Enables you to use the `<hr>` HTML tag in the editor.
 ## Commands
 | Command         | Options | Description               |
 | --------------- | ------- | ------------------------- |
-| horizontal_rule | —       | Create a horizontal rule. |
+| horizontalRule | —       | Create a horizontal rule. |
 
 ## Keyboard shortcuts
 *None*
 
+## Source Code
+[packages/extension-horizontal-rule/](https://github.com/ueberdosis/tiptap-next/blob/main/packages/extension-horizontal-rule/)
+
+## Usage
+<demo name="Extensions/HorizontalRule" highlight="3-5,17,36" />
+
 ## Usage
 ```markup
 <template>