Document how to download data & use nowcasting_dataset

[JackKelly]

Detailed Description

Provide a list of instructions for how to:

Download:

• NWPs
• EUMETSAT data
• PV data from PVOutput.org
• GSP-level PV
• Topographical data

Pre-process:

• NWPs
• EUMETSAT data
• PV data from PVOutput.org
• GSP-level PV
• Topographical data

The disk space required for:

• NWPs native
• NWPs intermediate (Zarr?)
• EUMETSAT native data
• EUMETSAT intermediate (Zarr?)
• PV data from PVOutput.org
• GSP-level PV
• Topographical data

Generate:

• Sun data

Also:

• Configure and run scripts/prepare_ml_data.py
• How to use the prepared data in an ML model (Document how to train an ML model using the pre-prepared batches. nowcasting_dataloader#5)

Possible Implementation

For now, we could just put this all into the README

[JackKelly]

I've added some documentation for downloading NWPs, satellite and PVOutput.org data in commit 537e45d