Added backup/restore guide

[azhavoro]

Motivation and context

resolve #2591

How has this been tested?

Manually

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 increased (+1.7%) to 75.412% when pulling a4d872c on az/backup_guide into 83a08cb on develop.

[nmanovic]

@azhavoro , please add a line into CHANGELOG.md