Improve versioning of the documentation website and fix page 404

[TOsmanov]

Motivation and context

This PR contains:

• improved versioning of the documentation website:
  • adds the display of the current version in the version selection menu
  • enable display the version menu on the mobile version of the site.
  • code from navbar_selector_menu.html move to navbar.html. Its done to correct display of the version menu on mobile version website.
  • version menu visible only on documentation pages
  • docs and images folders are now deleted before git checkout because files that are not in the previous version remain after the site is generate ( example of the unwanted page: https://openvinotoolkit.github.io/cvat/v1.5.0/docs/manual/basics/tasks-page/ )
• fix styles of the page 404

Preview site

https://tosmanov.github.io/cvat/

preview page 404:

https://tosmanov.github.io/cvat/docs/123
https://tosmanov.github.io/cvat/docs/123/456

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]

I feel, the contributing section is not supposed to be always unfold, but now it is.

[TOsmanov]

I feel, the contributing section is not supposed to be always unfold, but now it is.

At the moment, the Contributing section does not have a division into basic and advanced subsections. All contributing sections are at level 2 they are always be visible. I see three solutions:

1. Comment out or delete the ul_show = 2 parameter in config.toml. In this case, only the Getting Started, Management, Administration, Contributions sections will always be visible, and basic and advanced subsections will only appear on the open section.
2. Divide the contribution into basic and advanced.
3. Merge Contributing subsections into a single file

Hiding the Contributing section and creating a link to the section in the menu is not suitable because in this case the subsections do not appear in the left sidebar menu