Update User Guide ( change label shortcuts )

[TOsmanov]

Motivation and context

This PR contains update item Labels, Shortcuts and images.
This PR resolved issue #3070

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Coverage remained the same at 73.998% when pulling ab49ea4 on TOsmanov:UG-update-num-key-shortcuts into 7c5d71d on openvinotoolkit:develop.

[bsekachev]

Suggested change

	You can change the label of an object with the help of hot keys. First, you need to assign a number (from 0 to 9) to labels.
	You can change the label of an object using hot keys. In order to do it, you need to assign a number (from 0 to 9) to labels. By default numbers 1,2...0 are assigned to the first ten labels.

[bsekachev]

Suggested change

	To do this, click on the button next to a label on the sidebar.
	To assign a number, click on the button placed at the right of a label name on the sidebar.