[ahmedmohiduet]

Closes #286 (part of #276)

Adds docs/notebooks/data/Pangeo_CMIP6_ML.ipynb, ML demo notebook for PangeoCMIP6 accessor from #284.

Covers:

• config cell: variables list plus start/end dates per split
• data pipeline: PangeoCMIP6, ConvertUnits, Drop, Deviation, TemporalRetrieval (delta_unit='month'), ToNumpy, Squeeze, Cache
• monthly DateRange train and validation splits
• small CNN trained on CPU via Train and PipelineLightningDataModule
• prediction plot against target

Also adds row in Gallery.ipynb and .gitignore entry for training output.

Notes:

• Depends on Create ArchiveIndex derived accessor class #284 and Demo Notebook #285, both unmerged. Branch has both merged in, so diff includes their commits until they land in develop.
• Executed in full on kernel pet_nb_cpu. Outputs saved, no errors.
• Needs network access for Google Cloud storage.
• Demo only: 3 epochs, so prediction is rough and plot shows cold bias.
• Normalisation uses fixed norm_mean and norm_std values.