Spreadsheet: Add support for example views and hyperlinks in the docs

Now the functions can actually be demonstrated by small examples,
embedded right inside the documentation via:
spreadsheet://example/<page>#<example_name>

Also allows pages to link to each other via the same scheme:
spreadsheet://doc/<page>

Applications/Spreadsheet/HelpWindow.h

@@ -51,7 +51,7 @@ public:
 
 private:
     static RefPtr<HelpWindow> s_the;
-    String render(const GUI::ModelIndex&);
+    String render(const StringView& key);
     HelpWindow(GUI::Window* parent = nullptr);
 
     JsonObject m_docs;