Remove S3 data access guidance for Pf8

[nw20]

These edits remove guidance for accessing Pf8 data via Sanger S3 storage.

1. Remove mention of S3 from docs/cloud_data_access/parasite_cloud_data_access.ipynb
2. Remove examples using S3 data from docs/pf8/Data_access.ipynb
3. Add info banner at head of docs/pf8/Data_access.ipynb explaining these ongoing changes

NOTE: until Pf8 snp-only zarr can be stored on GCS and integrated into malariagen_data package, we will maintain an example accessing via S3. This will continue to work until the snp-only data is removed from S3. If/when snp-only zarr is available via GCS, we can make another update to docs/pf8/Data_access.ipynb.

These changes are ready to merge once we have removed S3 URLs from malariagen_data

[nw20]

S3 URL removed as default from malariagen_data.Pf8() - see this PR