Update item Annotation with polygons in User guide

[TOsmanov]

Motivation and context

This PR contains update of the text, images and gif of the Annotation with polygons item.
The item "Annotation with auto-segmentation" is transferred to the item "Make AI polygon".
And also adds description drawing using automatic borders.

The following measures were taken to solve problem #1447:

gif005_DETRAC.gif and gif009_DETRAC.gif has a reduced size.

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 5319

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 190 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.05%) to 65.701%

---

Files with Coverage Reduction	New Missed Lines	%
src/annotations-objects.js	4	58.76%
cvat/apps/dataset_manager/bindings.py	5	93.24%
cvat/apps/engine/media_extractors.py	5	77.65%
cvat/apps/log_viewer/views.py	6	0%
datumaro/datumaro/cli/main.py	31	0%
cvat/apps/auto_segmentation/views.py	60	0%
cvat/apps/git/git.py	79	16.12%

Totals
Change from base Build 5163:	0.05%
Covered Lines:	10847
Relevant Lines:	16109

---

💛 - Coveralls

[nmanovic]

@TOsmanov , could you please check that all images are visible next time (press view file and inspect it)? Image file names are case sensitive. I will recommend to rename all files in lowercase.

P.S. You probably don't see the issue on Windows because file names on Windows are case insensitive. Thus you need to check the user_guide.md here.

[TOsmanov]

@nmanovic , yes it is, I corrected the file names and paths in PR #1596 , to them so that they did not contain uppercase letters.

[nmanovic]

@TOsmanov , need to fix the PR. After you renamed all files it isn't correct anymore.

[nmanovic]

LGTM

[nmanovic]

@TOsmanov , need to resolve conflicts.

[TOsmanov]

@nmanovic , one of the conflict files is "gif009_detrac.gif", I don’t know how to resolve such a conflict

[TOsmanov]

@nmanovic , I created a new PR #1612 , it has no conflicts