Add "Documentation" button to the "help" menu (#2381)

docs/source/user-docs/menus/menu-bar/help-menu.rst

@@ -11,4 +11,10 @@ Report an Issue
 ----------------------------------------
 **Description:** Quickly report an issue to Cutter's GitHub repository. Clicking this option will navigate your browser to the new-issue page in Cutter's GitHub repository. It will also automatically fill relevant information inside the issue template.    
 
-**Steps:** Help -> Report an issue
+**Steps:** Help -> Report an issue
+
+Documentation
+---------------------------------------
+**Description:** Clicking this option will open the user documentation of Cutter in your browser.
+
+**Steps:** Help -> Documentation

src/core/MainWindow.cpp

@@ -232,6 +232,7 @@ void MainWindow::initUI()
   
     connect(ui->actionSaveLayout, &QAction::triggered, this, &MainWindow::saveNamedLayout);
     connect(ui->actionManageLayouts, &QAction::triggered, this, &MainWindow::manageLayouts);
+    connect(ui->actionDocumentation, &QAction::triggered, this, &MainWindow::documentationClicked);
 
     /* Setup plugins interfaces */
     for (auto &plugin : Plugins()->getPlugins()) {
@@ -1579,6 +1580,11 @@ void MainWindow::on_actionIssue_triggered()
     openIssue();
 }
 
+void MainWindow::documentationClicked()
+{
+    QDesktopServices::openUrl(QUrl("https://cutter.re/docs/user-docs"));
+}
+
 void MainWindow::on_actionRefresh_Panels_triggered()
 {
     this->refreshAll();

src/core/MainWindow.h

@@ -164,6 +164,7 @@ public slots:
 private slots:
     void on_actionAbout_triggered();
     void on_actionIssue_triggered();
+    void documentationClicked();
     void addExtraGraph();
     void addExtraHexdump();
     void addExtraDisassembly();

src/core/MainWindow.ui

@@ -131,6 +131,7 @@
     </property>
     <addaction name="actionAbout"/>
     <addaction name="actionIssue"/>
+    <addaction name="actionDocumentation"/>
    </widget>
    <widget class="QMenu" name="menuEdit">
     <property name="title">