Help: Add a simple index page

Let's show something a bit more welcoming than empty white when the user
launches the Help application. :^)

Applications/Help/main.cpp

@@ -287,6 +287,10 @@ int main(int argc, char* argv[])
             auto& search_model = *static_cast<GUI::FilteringProxyModel*>(model);
             search_model.set_filter_term(search_box.text());
         }
+    } else {
+        String path = "/usr/share/man/man7/Help-index.md";
+        history.push(path);
+        open_page(path);
     }
 
     window->set_focused_widget(&left_tab_bar);

Base/usr/share/man/man7/Help-index.md

@@ -0,0 +1,15 @@
+![Help icon](file:///res/icons/32x32/app-help.png)
+
+## Welcome to the SerenityOS on-line help system!
+
+This is **Help**, the built-in documentation viewer for the SerenityOS desktop environment. If you prefer a command-line interface, the [man](../man1/man.md) command offers a text-only view of the same library.
+
+---
+
+Documentation is divided into traditional Unix-style manual categories. Use the **Browse** tab on the left-hand side to navigate by category.
+
+There's also a full-text search option under the **Search** tab.
+
+---
+
+Thank you for using ***SerenityOS Help!***