[guide] Add guide to menu

Author: anjakefala

visidata/guides/MenuGuide.md

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+# The Menu System
+
+Located at the very top of the screen, the menu organizes many commands within submenu trees.
+
+Navigate the menu like a GUI by using the mouse to click on menu items.
+
+To navigate with the keyboard, `Alt+H` will open the **Help** menu, and from there use the arrow keys to move between submenus.
+
+Additionally, each top-level menu name has a single letter underlined. `Alt+<underlined letter>` will open that menu. For example, `Alt+F` will open the **File** menu.
+
+To open a submenu (indicated by `{vd.options.disp_menu_more}`) press the `→` key or click on it with the mouse. Each non-submenu item is a command.
+
+The menu shows the description of the current command, along with the command name and its keyboard shortcut (if available). Execute the current command by clicking on it or pressing `Enter`.
+
+Some other indicators:
+
+- `{vd.options.disp_menu_push}`: the command opens another sheet.
+- `{vd.options.disp_menu_input}`: the command requires input.
+
+Some commands are only available on a specific sheet type, and appear [:color_menu_spec]highlighted[/] in the menu when the current sheet is that type. If the command is not available, it will not appear at all.
+
+## Options
+
+- {help.options.disp_menu}