Add item Track mode with polygons in User Guide

[TOsmanov]

This PR contains adding item Track mode with polygons.
Also fixed typos in keyboard shortcut.

Motivation and context

N/A

How has this been tested?

N/A

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6596

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 64.976%

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Totals
Change from base Build 6575:	0.03%
Covered Lines:	11088
Relevant Lines:	16658

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💛 - Coveralls

[nmanovic]

LGTM