Add menu keybindings to documentation

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.El

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.Ss MENU KEYS

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The following keys bindings are used for the feh menu:

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.Bl -tag -width indent

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.It Ao Escape Ac Bq menu_close

Close the menu

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.It Ao Up Ac Bq menu_up

Highlight previous menu item

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.It Ao Down Ac Bq menu_down

Highlight next menu item

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.It Ao Left Ac Bq menu_parent

Highlight parent menu item

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.It Ao Right Ac Bq menu_child

Highlight child menu item

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.It Ao Return Ac , Ao Space Ac Bq menu_select

Select highlighted menu item

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.El

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.Sh MOUSE ACTIONS

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