[Docs] errors in xarray page

[callumrollo]

When working through the xarray page of the docs, I encountered a few errors/omissions that added friction to my quick attempt at using icechunk + xarray:

• [x] import line imports modules IcechunkStore, StorageConfig from icechunk; later icechunk.StorageConfig is called, which fails as icechunk itself was not imported.
• [x] ds = xr.tutorial.open_dataset('rasm') requires the packages pooch and netCDF4 to be installed, this should be included in the instructions
• [x] Final line states that Notice that this second xarray.Dataset has a time dimension of length 18 whereas the first has a time dimension of length 36, but the first has dimenstions (y: 205, x: 275, time: 18). I think this is just a copy-paste error when making the docs

I will submit 3 PRs to remedy these. Looking forward to working with this package!

[paraseba]

Closed by @callumrollo PRs. Thank you.