documentation update

[CarloLucibello]

I factored out here the part of #2412 involving only the documentation. Most of it is removing any reference to Flux.params and to the implicit differentiation style.

I also reorganized a bit files and folders, updated and reintroduced the old mlp tutorial, and update to the last Documenter version.

Needs FluxML/Optimisers.jl#174 to be merged and tagged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.13%. Comparing base (caa1cee) to head (15727cb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2422   +/-   ##
=======================================
  Coverage   46.13%   46.13%           
=======================================
  Files          32       32           
  Lines        1877     1877           
=======================================
  Hits          866      866           
  Misses       1011     1011           

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[github-actions]

Once the build has completed, you can preview any updated documentation at this URL: https://fluxml.ai/Flux.jl/previews/PR2422/ in ~20 minutes

[mcabbott]

I'm against restoring the tutorial in this form. All this text is not very helpful. You can't easily play with pieces of this to see how they work.

The model zoo has a re-write, and it could plausibly be tidied up to live here too:

https://github.com/FluxML/model-zoo/blob/master/vision/mlp_mnist/mlp_mnist.jl

I agree it would be nice if something like this were easier to find. The problem we never solved with moving model zoo models in here was that they aren't tested and tend to rot, so this means two things to check manually.

[CarloLucibello]

I put away the MLP tutorial (along with an updated 60min-blitz) and created a "Model Zoo" page instead in the tutorial hopefully helping a bit the discoverability of the model zoo examples.

[CarloLucibello]

merge?

[mcabbott]

Would like to look over the rest but not today, sorry