Documentation fixes

[racousin]

Fix Implementation

Navigation Renaming

Before:
├── Getting Started
├── User Guide
├── Contributing
└── ArchesWeather section
After:
├── Installation & Setup
├── User Guide & Tutorials
├── Models (ArchesWeather & ArchesWeatherGen)
└──Contributing

Add Missing Setup Information

1. Install GeoArches

cd geoarches
poetry install

2. Download Sample Data
Required Step: Download ERA5 data to test the package:
bashpython geoarches/download/dl_era.py --years 2020
This command downloads ERA5 reanalysis data for 2020, which is needed for training and testing models.
Note: Data download may take 15-30 minutes depending on your connection.

[14renus]

Note: 2020 can be used to test but for training, if domain='train', dataloader will try to load train split 1979-2018.

https://github.com/INRIA/geoarches/blob/main/geoarches/dataloaders/era5.py#L19C5-L20C68