Extend generator script user guide

docs/source/api/generation.rst

docs/source/conf.py

docs/source/index.md

...es/guide_generator_scripts/custom_cmdline_args/kernels.py

...examples/guide_generator_scripts/inline_config/kernels.py

docs/source/usage/generator_scripts.md

docs/source/usage/project_integration.md

pyproject.toml

src/pystencilssfg/composer/basic_composer.py

src/pystencilssfg/generator.py