update documentation

[sophiamaedler]

We need to relocate the doc files to fit with the scverse standard

completed in #98

[sophiamaedler]

Reformat docs to the following structure:

• installation
• introduction
• workflow
• tutorials

completed in #105

[sophiamaedler]

Update documentation to reflect new code base and to improve user experience:

• move notebooks to separate repo (https://github.com/MannLabs/scPortrait-notebooks) → this will make this repo easier to maintain
• integrate notebook repo with binder
• add example notebooks to documentation
• add section for code snippets
• document stitching code

[sophiamaedler]

created structure to display code examples in gallery and include notebooks from separate repo in #113
Will comment out new changes in display of documentation and move these to a separate branch until all notebooks are available. Needed to merge now to avoid issues with the inclusion of the submodule.