Re-organize the notebook hierarchy

[ablaom]

This PR put the two series of tutorials in separate folders:

$ tree notebooks
notebooks
├── learning_curve.svg
├── lightning_tour
│   ├── notebook.jl
│   ├── notebook.md
│   └── Project.toml
├── MLJTutorial
│   ├── 01_data_representation
│   │   ├── notebook.jl
│   │   ├── notebook.md
│   │   ├── Project.toml
│   │   └── scitypes.svg
│   ├── 02_models
│   │   ├── learning_curve.png
│   │   ├── notebook.jl
│   │   ├── notebook.md
│   │   └── Project.toml
│   ├── 03_pipelines
│   │   ├── notebook.jl
│   │   ├── notebook.md
│   │   └── Project.toml
│   ├── 04_tuning
│   │   ├── gamma_sampler.png
│   │   ├── learning_curve.png
│   │   ├── learning_curve2.png
│   │   ├── neural_net.jls
│   │   ├── notebook.jl
│   │   ├── notebook.md
│   │   ├── Project.toml
│   │   └── tuning.png
│   ├── 05_composition
│   │   ├── notebook.jl
│   │   ├── notebook.md
│   │   └── Project.toml
│   └── 99_solution_to_exercises
│       ├── exercise_6ci.png
│       ├── exercise_7c.png
│       ├── exercise_8c.png
│       ├── notebook.jl
│       ├── notebook.md
│       └── Project.toml
├── tuned_model_grid.svg
├── tuned_model_random.svg
└── UsingMLJ
    ├── 01_basics
    │   ├── notebook.jl
    │   ├── notebook.md
    │   └── Project.toml
    ├── 02_model_composition
    │   ├── notebook.jl
    │   ├── notebook.md
    │   ├── Project.toml
    │   └── runtests.jl
    └── 03_model_tuning
        ├── learning_curve.svg
        ├── notebook.jl
        ├── notebook.md
        ├── Project.toml
        ├── runtests.jl
        ├── tuned_model_grid.svg
        └── tuned_model_random.svg

CI is likely to fail in the the auto-generated markdown workflow, which can be ignored.

[github-actions]

Attention

A notebook or Project.toml file has changed and new notebook
markdown has been generated. After merging this pull request, review #63
to commit the changes and update the documentation on gh-pages, as this will
need to be regenerated.