feat[next]: workshop material and docs field Dims type hint update

[nfarabullini]

edited type hints in gt4py/docs/user/next from e.g.:
Field[[CellDim, KDim], dtype]
to:
Field[Dims[CellDim, KDim], dtype]

Addition of all workshop material under the same folder

[egparedes]

All the changes look good to me. The only missing part is to add automatic checking of all the new notebooks (and the quickstart guide if it's not already tested) to make sure they don't go out of sync with the code.

[egparedes]

It looks good to me, but please extend the current PR title and description to include the description of all the added changes in the commit.