Add GetPanel(), GetPanelById() and GetPanelCount() to ribbon page

This provides parity with the ribbon bar's GetPage() set of functions,
and adds an easy way to find a panel and show/hide it.

Also, add example and discussion about showing/hiding panels on a
ribbon.

Closes #24319.

include/wx/ribbon/page.h

@@ -62,6 +62,10 @@ public:
 
     void HideIfExpanded();
 
+    wxRibbonPanel* GetPanel(int n);
+    wxRibbonPanel* GetPanelById(wxWindowID id);
+    size_t GetPanelCount();
+
 protected:
     virtual wxSize DoGetBestSize() const override;
     virtual wxBorder GetDefaultBorder() const override { return wxBORDER_NONE; }