[ENH] Forecasting refactor

[TonyBagnall]

Reorganise the forecasting module as follows

forecasting
--- stats
ETS, ARIMA
--- deep_learning (when we have some)
--- machine_learning (when we have some)
--- utils (including nelder mead)
--- tests

Also rename ETSForecaster as ETS

edit: when sorting out the API docs I noticed that there were still incorrect references to registry, so also changed them here

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I have added the following labels to this PR based on the title: [ enhancement ].
I would have added the following labels to this PR based on the changes made: [ forecasting ], however some package labels are already present.

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[MatthewMiddlehurst]

GitHub is showing some of these as new files rather than moved i.e. ARIMA and the utils. Can you check that these arent duplicates or contents from another PR

[TonyBagnall]

GitHub is showing some of these as new files rather than moved i.e. ARIMA and the utils. Can you check that these arent duplicates or contents from another PR

that will be because I branched this of the ARMA branch I assume

[alexbanwell1]

Looks fine now