Modernization

[ablaom]

The tutorials are now more than 5 years old, and are based on some outdated API, especially around model composition. They do not run under Julia 1.12.

In this substantial PR we:

• Update tutorials to reflect the latest API, and to enable them to run on julia 1.12
• For accessibility and maintainability, tutorials are now integrated into a single Documenter.jl generated web-page (they are moved to /docs/src/notebooks). However, the ground truth for each tutorial is still a julia script, annotated so that Literate.jl can generate the required markdown (with execute=true). New CI changes completely automate this markdown generation process. If only a single tutorial changes, then only that tutorial's markdown will be regenerated. Package management uses Julia's newer workspaces structure. There are no committed manifests, but each tutorial get's it's own Project.toml file which include [compat] lower bounds for all packages needed in that tutorial. If any Project.toml file in the workspace tree changes, all tutorials get regenerated. For more detail see the updated README.md.
• Deprecate the Pluto tutorials. I simply don't have the bandwidth to keep these synchronised and automating this in not currently straightforward, as far as I can tell. (Plus I now have "recycled" variable names in the tutorials which Pluto will not tolerate). The old pluto notebooks, which presumably still run under Julia 1.6 are moved to /pluto_notebooks but tagged as "legacy" files.

To do:

• The tutorials that use EvoTrees models will need to be updated when Fix MLJ update to refit when hyperparameters other than nrounds change Evovest/EvoTrees.jl#347 is merged.