Add links to docs in the Help menu (#3274)

* Fix #3266 and Fix #3197 
* Add documentation links to Help menu
* Added offline documentation (PDF) for Getting Started Guide and User Guide

src/gui/MainWindow.h

@@ -91,6 +91,9 @@ private slots:
     void hasUpdateAvailable(bool hasUpdate, const QString& version, bool isManuallyRequested);
     void openDonateUrl();
     void openBugReportUrl();
+    void openGettingStartedGuide();
+    void openUserGuide();
+    void openOnlineHelp();
     void switchToDatabases();
     void switchToSettings(bool enabled);
     void switchToPasswordGen(bool enabled);
@@ -132,6 +135,7 @@ private:
     bool saveLastDatabases();
     void updateTrayIcon();
     bool isTrayIconEnabled() const;
+    void customOpenUrl(QString url);
 
     static QStringList kdbxFilesFromUrls(const QList<QUrl>& urls);
     void dragEnterEvent(QDragEnterEvent* event) override;