Document how to use Taurus lateral applet bar.
Taurus right lateral applet/widget bar in TaurusGUIs is not documented.
Documentation to be added in Taurus:
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Widgets can be added to applet/widget bar by defining this lines in the config.py of the corresponding taurusgui:
cd = AppletDescription('CountDownWidget',
classname = 'countdown.CountDownWidget')
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