Removed Need help gitter link

[klakhov]

Motivation and context

Gitter channel is not available anymore

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see 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.

[KTXKIKI]

or Can change it to redirect to YouTube for easy viewing of teaching videos? https://www.youtube.com/@cvat-ai/featured

[codecov]

Codecov Report

Merging #7002 (82bb99a) into develop (1b77005) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #7002      +/-   ##
===========================================
+ Coverage    82.60%   82.61%   +0.01%     
===========================================
  Files          360      360              
  Lines        38922    38922              
  Branches      3570     3570              
===========================================
+ Hits         32150    32156       +6     
+ Misses        6772     6766       -6     

Components	Coverage Δ
cvat-ui	77.64% <ø> (+0.03%)	⬆️
cvat-server	87.17% <ø> (ø)