docs: help pages for cursors, measurements and math channels

src/help_window.cpp

@@ -38,6 +38,9 @@ constexpr HelpPage kPages[] = {
     {"Viewing", "Navigating the plot", "/org/udpscope/help/06-navigate.txt"},
     {"Viewing", "Pause and play", "/org/udpscope/help/07-pause.txt"},
     {"Viewing", "Trigger", "/org/udpscope/help/08-trigger.txt"},
+    {"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"},
 };
 constexpr int kPageCount = static_cast<int>(sizeof(kPages) / sizeof(kPages[0]));