Doc: Describe how to hide and show the main menu bar

After users hide the menubar, they cannot see the keyboard
shortcut for showing it anymore.

Task-number: QTCREATORBUG-29392
Change-Id: I7a26d6bd0972457e225b33d2b041c158ad71548e
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

doc/qtcreator/src/user-interface/creator-only/creator-how-to-show-and-hide-main-menu.qdoc

@@ -0,0 +1,22 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
+
+/*!
+    \page creator-how-to-show-and-hide-main-menu.html
+    \previouspage creator-how-tos.html
+
+    \ingroup creator-how-to-ui
+
+    \title Show and hide the main menu
+
+    On Linux and Windows, you can hide the main menu bar to save space on the
+    screen. Select \uicontrol View, and deselect the \uicontrol {Show Menubar}
+    check box.
+
+    \image qtcreator-without-menubar.webp {Qt Creator without the main menu}
+    \caption \QC without the main menu bar.
+
+    To show the main menu again, press \key {Ctrl+Alt+M}.
+
+    \sa {Show and hide sidebars},{Switch between modes}
+*/

doc/qtcreator/src/user-interface/creator-only/creator-how-to-show-and-hide-sidebars.qdoc

@@ -35,5 +35,5 @@
     In some views, right-clicking opens a context menu that has functions
     for managing the objects listed in the view.
 
-    \sa {Sidebar Views}
+    \sa {Sidebar Views},{Show and hide the main menu},{Switch between modes}
 */

doc/qtcreator/src/user-interface/creator-only/creator-how-to-switch-between-modes.qdoc

@@ -72,4 +72,6 @@
     \uicontrol View > \uicontrol {Mode Selector Style} > \uicontrol Hidden.
     To only show icons on the mode selector, select the \uicontrol {Icons Only}
     style.
+
+    \sa {Show and hide sidebars},{Show and hide the main menu}
 */