[DOC] add explanation of dunder method construction and postprocessing in pipelines to forecasting tutorial

[fkiraly]

An explanation of "compact" dunder pipelining with TransformedTargetForecaster should be added to the forecasting tutorial - not replacing, but being added to the subsections where pipelines are explained.

For example, instead of

pipe = TransformedTargetForecaster(
    steps=[
        ("imputer", Imputer()),
        ("forecaster", UnobservedComponents())
    ]
)

one can also write

pipe = Imputer() * UnobservedComponents()

This would also be a great opportunity to explain postprocessing, i.e., transformers can also be added after the forecaster since 0.11.2.

[nshahpazov]

@fkiraly I would like to add this. There's already documentation on the dunder method usage, but nothing about post-processing.

[fkiraly]

thanks!