Delete Intro to Dask Notebook in favor of dask docs

[scottyhq]

#322 pointed out there there is both a intro to dask notebook in the "fundamentas" section that is not listed in the TOC and therefore not on the website:
https://github.com/xarray-contrib/xarray-tutorial/blob/main/fundamentals/05_intro_to_dask.ipynb

As well as this "intermediate" one that is rendered https://tutorial.xarray.dev/intermediate/xarray_and_dask.html

I propose we just remove it (there was discussion in the past about just removing most content focusing on dask here to link to dask docs or other dask tutorials):
#65 (comment)