update COCO format documentation

[zmaslova]

Motivation and context

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

[zhiltsov-max]

@azhavoro, do we need to sync with the docs site?

[nmanovic]

@zmaslova , could you please update your PR? We have added a special site for our documentation: https://openvinotoolkit.github.io/cvat/docs/

[zmaslova]

@zmaslova , could you please update your PR? We have added a special site for our documentation: https://openvinotoolkit.github.io/cvat/docs/

Updated

[nmanovic]

@zmaslova , there is a problem in the patch. Could you please fix it?

[zmaslova]

@zmaslova , there is a problem in the patch. Could you please fix it?

Fixed. I also see issues in CHANGELOG.md, I suppose I shouldn't fix them