Add intermediate remote data access tutorial

Illustrate
- [ ] opendap / thredds
- [ ] datasets on S3
- [ ] datasets using gcsfs
- [ ] fsspec

Example code:
``` python
import gcsfs

fs = gcsfs.GCSFileSystem(token="anon")
ds = xr.open_zarr(
    fs.get_mapper("gs://pangeo-noaa-ncei/noaa.ersst.v5.zarr"), consolidated=True
).load()
ds
```