CI: Temporarily pin geopandas<1.0 in the Docs workflow

[seisman]

In 45ff5a4, I set the parameter engine="pyogrio" so that geopandas would use pyogrio instead of fiona.

• geopandas 0.14 + pyogrio 0.9 works (https://github.com/GenericMappingTools/pygmt/actions/runs/9678856324/job/26703528256?pr=3304)
• geopandas 1.0 + pyogrio 0.9 fails (https://github.com/GenericMappingTools/pygmt/actions/runs/9678923611/job/26703733741?pr=3304)

The conda environments diff a little:

diff gpd-0.14.txt gpd-1.0.txt
61d60
<     fiona                          1.9.6         py312h32ad294_3          conda-forge
76,77c75,76
<     geopandas                      0.14.4        pyhd8ed1ab_0             conda-forge
<     geopandas-base                 0.14.4        pyha770c72_0             conda-forge
---
>     geopandas                      1.0.0         pyhd8ed1ab_0             conda-forge
>     geopandas-base                 1.0.0         pyha770c72_0             conda-forge
144d142
<     libspatialindex                2.0.0         he02047a_0               conda-forge
220d217
<     rtree                          1.2.0         py312h3ed4c40_1          conda-forge

So it's likely something wrong with geopandas 1.0, but not sure why I can't reproduce it locally. The Tests workflow also have geopandas 1.0 installed but all tests pass (see https://github.com/GenericMappingTools/pygmt/actions/runs/9671820457/job/26701186881).

Maybe we just need to wait for a few days and see if there are any upstream fixes/changes. This PR pins geopandas<1.0 in the Docs workflow so that the Docs build passes.

Address #3301