add a demo to the bullet list extension page

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

@@ -0,0 +1,12 @@
+context('/api/extensions/bullet-list', () => {
+  before(() => {
+    cy.visit('/api/extensions/bullet-list')
+  })
+
+  beforeEach(() => {
+    cy.get('.ProseMirror').then(([{ editor }]) => {
+      editor.setContent('<p>Example Text</p>')
+      editor.selectAll()
+    })
+  })
+})