On Windows and Linux, use Ctrl where this page says Cmd.

ShortcutAction
Cmd + BBold
Cmd + IItalic
Cmd + UUnderline
Cmd + Shift + SStrikethrough
Cmd + KAdd or edit link
Alt/Option + Shift + LAlign left
Alt/Option + Shift + CAlign center
Alt/Option + Shift + RAlign right
Cmd + ZUndo
Cmd + Shift + ZRedo
/Open the widget menu

To see these inside the editor, type /help.

For markdown shortcuts such as ## for a heading, see Write in the editor.