Add Automated Dataset Download Script for Easy Access

[JVSCHANDRADITHYA]

This PR introduces an automated dataset download script to simplify the process of obtaining the National Gallery of Art Open Dataset. The following additions have been made:

**download_dataset.py** : A Python script that downloads and stores the dataset efficiently using the IIIF API.

**download_dataset.sh** : A Bash script for Linux/macOS users to automate the download process.

**download_dataset.bat** : A Batch script for Windows users to enable easy dataset retrieval.

Updated **README.md** : Added clear instructions on how to use the scripts to download the dataset.