Add usage manual for VisualDL, and modify four pictures in directory demo/component/usage_interface. (#549)

The usage maunal will be helpful for beginners to make use of VisualDL.

The components of ViusalDL are introduced in details,
including definitions of functions, parameters explanations.
Moreover, demos are given for each component.

The former pictures of demo are lack of some details in the frontend webpage,
I upload new pictures to make demo more intuitive.