Refactor and Improve Documentation

.gitlab-ci.yml

doc/conf.py

doc/img/c_backend.svg

doc/img/github_repo_card.png

doc/img/logo.png

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doc/img/logo_large.svg

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doc/img/pystencils_arch_block_diagram.svg

doc/img/pystencils_stencil_four_points_with_arrows.svg

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doc/index.rst

doc/notebooks/01_tutorial_getting_started.ipynb

doc/notebooks/02_tutorial_basic_kernels.ipynb

doc/notebooks/03_tutorial_datahandling.ipynb

doc/notebooks/04_tutorial_advection_diffusion.ipynb

...books/05_tutorial_phasefield_spinodal_decomposition.ipynb

doc/notebooks/06_tutorial_phasefield_dentritic_growth.ipynb

doc/notebooks/demo_assignment_collection.ipynb