Add color map conversion example in docs

[zhiltsov-max]

Motivation and context

Added an example of a feature to change mask colors using Datumaro. Such questions are raise quite often the last time.

How has this been tested?

Manual test

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 5634

• 2 of 13 (15.38%) changed or added relevant lines in 2 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.08%) to 65.758%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/plugins/voc_format/converter.py	1	6	16.67%
datumaro/datumaro/util/init.py	1	7	14.29%

Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	7	75.59%

Totals
Change from base Build 5621:	-0.08%
Covered Lines:	10913
Relevant Lines:	16203

---

💛 - Coveralls