Extend SDK layer 1 docs

[zhiltsov-max]

Motivation and context

• Added more documentation on ApiClient usage
• Added autogeneration of github pages for ApiClient members
• Refactored GitHub workflows to generate SDK only once

How has this been tested?

mkdir -p cvat-sdk/schema/ && python manage.py spectacular --file cvat-sdk/schema/schema.yml
cd cvat-sdk/
gen/generate.sh

cd ..

git submodule update --init --recursive
site/process_sdk_docs.py
cd site/
hugo server

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.

[SpecLad]

Frankly, with so much fixing up I wonder if it would be simpler to write a custom documentation generator...

[zhiltsov-max]

@nmanovic , this PR is ready for merging.