Docs building: remove the two environments creation

[martinRenou]

For some reason I don't understand, we are now creating two environments when building the docs:

• https://github.com/geojupyter/jupytergis/blob/main/.readthedocs.yaml#L24
• https://github.com/geojupyter/jupytergis/blob/main/.readthedocs.yaml#L26

It's unclear to me why the rtd setup is so complicated, we can simplify it and make our life easier

[mfisher87]

We have one environment for building JupyterGIS and another for building the docs. Doing this in a combined environment caused the docs build to fail (IIRC JupyterLite step?). I don't remember the error message, but I posted about it somewhere... Not sure where right now :)