remove notebooks from CI

[giovp]

• [x] convert all python files in tutorials into ipynb
• [x] run all new notebooks
• [x] move all new notebooks into docs/source/tutorials (create this new folder tutorials)
• [x] copy external_tutorials into new folder tutorials
• [x] in tutorials.rst add correct indexing for the documentation.
• [x] update tutorial.rst also in squidpy

[LLehner]

Completed in #113.