Add document insights panel (#4418)

* Add document context to allow accessing editor in header, modals, and elsewhere

* lint

* framework

* Hacking together fast

* Insights

* Spacing tweak, docs

shared/editor/marks/Placeholder.ts

@@ -13,6 +13,7 @@ export default class Placeholder extends Mark {
     return {
       parseDOM: [{ tag: "span.template-placeholder" }],
       toDOM: () => ["span", { class: "template-placeholder" }],
+      toPlainText: () => "",
     };
   }