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/React/styles.scss

@@ -0,0 +1,24 @@
+/* Basic editor styles */
+.ProseMirror {
+  > * + * {
+    margin-top: 0.75em;
+  }
+}
+
+/* Placeholder (at the top) */
+/*.ProseMirror p.is-editor-empty:first-child::before {
+  content: attr(data-placeholder);
+  float: left;
+  color: #ced4da;
+  pointer-events: none;
+  height: 0;
+}*/
+
+/* Placeholder (on every new line) */
+.ProseMirror .is-empty::before {
+  content: attr(data-placeholder);
+  float: left;
+  color: #ced4da;
+  pointer-events: none;
+  height: 0;
+}