Help+LibManual: Without arguments, open index page instead of crashing

This is the old behavior before the recent LibManual refactor. It also
moves the definition of the index page into LibManual for better reuse.

Userland/Applications/Help/MainWidget.cpp

@@ -213,7 +213,7 @@ ErrorOr<void> MainWidget::set_start_page(Vector<StringView, 2> query_parameters)
 
 ErrorOr<void> MainWidget::initialize_fallibles(GUI::Window& window)
 {
-    static String const help_index_path = TRY(TRY(try_make_ref_counted<Manual::PageNode>(Manual::sections[7 - 1], TRY(String::from_utf8("Help-index"sv))))->path());
+    static String const help_index_path = TRY(TRY(Manual::PageNode::help_index_page())->path());
     m_go_home_action = GUI::CommonActions::make_go_home_action([this](auto&) {
         m_history.push(help_index_path);
         open_page(help_index_path);

Userland/Libraries/LibManual/Node.cpp

@@ -25,8 +25,7 @@ ErrorOr<NonnullRefPtr<PageNode>> Node::try_create_from_query(Vector<StringView,
     auto query_parameter_iterator = query_parameters.begin();
 
     if (query_parameter_iterator.is_end())
-        // BUG! No query was given.
-        VERIFY_NOT_REACHED();
+        return PageNode::help_index_page();
 
     auto first_query_parameter = *query_parameter_iterator;
     ++query_parameter_iterator;

Userland/Libraries/LibManual/PageNode.cpp

@@ -7,6 +7,7 @@
 
 #include "PageNode.h"
 #include "SectionNode.h"
+#include <AK/RefPtr.h>
 
 namespace Manual {
 
@@ -26,4 +27,10 @@ ErrorOr<String> PageNode::path() const
     return TRY(String::formatted("{}/{}.md", TRY(m_section->path()), m_page));
 }
 
+ErrorOr<NonnullRefPtr<PageNode>> PageNode::help_index_page()
+{
+    static NonnullRefPtr<PageNode> const help_index_page = TRY(try_make_ref_counted<PageNode>(sections[7 - 1], TRY(String::from_utf8("Help-index"sv))));
+    return help_index_page;
+}
+
 }

Userland/Libraries/LibManual/PageNode.h

@@ -30,6 +30,8 @@ public:
 
     ErrorOr<String> path() const;
 
+    static ErrorOr<NonnullRefPtr<PageNode>> help_index_page();
+
 private:
     NonnullRefPtr<SectionNode> m_section;
     String m_page;