[Revise] change the default directory to save Natural Earth data

[lisphilar]

Summary of this new feature

As discussed in #1173, Natural Earth published under Public Domain, will be included in covsirphy as geopandas does.

• Add a script of downloading/updating the dataset to stable version publish workflow
• Save the file in covsirphy/gis/natural_earth directory with README.md (to explain the data source)
• Ensure covsirphy to include the dataset with pyproject.toml (using "include" option?)
• covsirphy.gis.geometry._Geometry._natural_earth() will read the saved files internally (with relative path from covsirphy directory?)
• Explain the workflow and data source in documentation

CovsirPhy version I used: 2.26.1

[lisphilar]

Because the dataset of the data source will be updated irregularly, the dataset will not be included in CovsirPhy. As a quick solution, the directory to save the files will be changed to "covsirphy/gis/Natural_Earth" directory.