Update workspaces URL logic and add documentation about this

[keller-mark]

From slack thread:
I want to make a request to
http://localhost:8019/A/0/4c460c95-d5ed-4b1f-abab-96f65d80b35a.sdata.zarr/images/expression/.zattrs
My notebook is running at
https://l004-pvt-hive-psc-edu-7652.workspaces-pt.hubmapconsortium.org/passthrough/lab/tree//blank.ipynb
So I need to rewrite my localhost URL to be the following:
https://l004-pvt-hive-psc-edu-8019.workspaces-pt.hubmapconsortium.org/A/0/4c460c95-d5ed-4b1f-abab-96f65d80b35a.sdata.zarr/images/expression/.zattrs

We need to update the logic here

vitessce-python/src/vitessce/widget.py

Line 254 in 92ace00

const isInWorkspaces = origin.startsWith(WORKSPACES_URL_KEYWORD);

[keller-mark]

We also need to clearly document the need to specify vc.widget(server_host="0.0.0.0) when using the widget on workspaces with data that is local to workspaces