Add xcube-eopf version of NDVI TimeSeries notebook

This notebook demonstrates the Dask Gateway + xcube-eopf approach for
processing Sentinel-2 and Sentinel-3 NDVI time series.

Note: Currently blocked by upstream issues:
- Issue #61: Missing proj:bbox in STAC metadata for older Sentinel-2 data
- Issue #62: Wrong coordinate arrays for Sentinel-3 OLCI (tie-point vs full-res)
- Issue #63: Broken asset URLs with /../ paths in newer Sentinel-3 data

The notebook includes debug cells documenting these issues and will work
once the upstream fixes are applied.

Author: Yuvraj198920