New example for custom documents (to force a heading on the top) (#1948)

* examples: add a custom document example, with a heading on top

* add link to the new example

* fix example page

demos/src/Examples/CustomDocument/Vue/index.spec.js

@@ -0,0 +1,7 @@
+context('/src/Examples/CustomDocument/Vue/', () => {
+  before(() => {
+    cy.visit('/src/Examples/CustomDocument/Vue/')
+  })
+
+  // TODO: Write tests
+})