Update readonly example and add test (#4212)

* fixes #4039, update readonly example

* add cypress test for readonly mode

cypress/integration/readonly.ts

@@ -0,0 +1,10 @@
+describe('readonly editor', () => {
+  it('should not be editable', () => {
+    cy.visit('examples/read-only')
+      .get('[data-slate-editor="true"]')
+      .should('not.have.attr', 'contentEditable', 'true')
+      .should('not.have.attr', 'role', 'textbox')
+      .click()
+      .should('not.be.focused')
+  })
+})

site/examples/read-only.tsx

@@ -17,7 +17,8 @@ const initialValue: Descendant[] = [
     type: 'paragraph',
     children: [
       {
-        text: 'This is editable plain text, just like a <textarea>!',
+        text:
+          'This example shows what happens when the Editor is set to readOnly, it is not editable',
       },
     ],
   },