start learning tutorial (#480)

* start learning tutorial

* reduce size

* change data link

* remove data files (they are hosted in NeurobloxDocsHost)

* cleanup

* cortico-striatal small circuit

* clean up

* minor style changes

* add number of trials argument to `ClassificationEnvironment` constructor

* reduce learning rate to avoid weight oscillations (negative values)

* update cortico-cortical plasticity eaxmple

* add before & after adjacency plots

* add trace to return trial ID and agent accuracy in `run_experiment!`

* update tutorial

* tune parameters for learning tutorial

* add text

* add single trial run

* add trial print line

* remove unrolled `run_trial!` and add `verbose` kwarg

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Co-authored-by: haris organtzidis <[email protected]>
Co-authored-by: Mason Protter <[email protected]>