& matthewmiddlehurst [DOC] improved docstring for dtw_distance

[fkiraly]

This improves the docstring of dtw_distance with mathematical detail, and adds related information to DtwDist.

Based on @MatthewMiddlehurst's PR #3852, with following changes:

• clarifies assumptions, e.g., equal length, only Euclidean distance
• links to more general time warping distance in sktime
• mathematic formalism is clarified and corrected in some places
• valuative formulations, e.g., "most researched" or "most common", are removed

[fkiraly]

failures unrelated to this PR, see #4033