DOC: Reader options underdocumented

[ianhi]

Issue

I was attempting to pass anon=True to s3fs via open_virtual_dataset with reader_options={"anon":True}

from virtualizarr import open_virtual_dataset

open_virtual_dataset(
    "s3://noaa-cdr-sea-surface-temp-optimum-interpolation-pds/data/v2.1/avhrr/201001/oisst-avhrr-v02r01.20100101.nc",
    indexes={},
    reader_options={"anon":True}
)

which fails because everything except for storage_options gets dropped here: https://github.com/zarr-developers/VirtualiZarr/blob/4906477415e74346703e970fbd81fe7ec0e12e81/virtualizarr/utils.py#L62

the correct argument for the behavior I want is reader_options={"storage_options":{"anon":True}}

which is briefly mentioned here: https://virtualizarr.readthedocs.io/en/stable/usage.html#opening-remote-files but not described.

Proposed fix

add something to the docstring mentioning the use of storage_options