docs: help pages for export, sessions and the status bar

src/help_window.cpp

@@ -41,6 +41,9 @@ constexpr HelpPage kPages[] = {
     {"Analysis", "Cursors", "/org/udpscope/help/09-cursors.txt"},
     {"Analysis", "Measurements", "/org/udpscope/help/10-measurements.txt"},
     {"Analysis", "Math channels", "/org/udpscope/help/11-math.txt"},
+    {"Output and state", "Saving and export", "/org/udpscope/help/12-export.txt"},
+    {"Output and state", "Sessions and settings", "/org/udpscope/help/13-sessions.txt"},
+    {"Output and state", "Status bar and diagnostics", "/org/udpscope/help/14-status.txt"},
 };
 constexpr int kPageCount = static_cast<int>(sizeof(kPages) / sizeof(kPages[0]));