Fix webhook signature

[sizov-kirill]

Motivation and context

• Delete necessary newline during generation of webhook signature
• Update documentation about webhooks secrets

How has this been tested?

Run code snippet that verifies webhook signature (see webhook documentation)

Checklist

• I submit my changes into the develop branch
• I have added a 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.