Add heatwave EFI 2t AWS notebook in opendata, update environment

[valtzen]

Description

Adds opendata/heatwave-efi-2t-aws.ipynb, a worked example of accessing and plotting the Extreme Forecast Index (EFI) for 2m temperature from the June 2026 European heatwave static dataset on AWS S3 (bucket ecmwf-static-datasets, DOI 10.5281/zenodo.21224650). Requested for the AWS Open Data Registry entry (awslabs/open-data-registry#3223).

The notebook uses the earthkit stack (earthkit-data + earthkit-plots) with anonymous S3 access via boto3. It covers the file structure, the EFI step-range convention (00 and 12 UTC runs), reading EFI against 2m temperature as a magnitude companion, and uses the standard ECMWF earthkit-plots styles.

Changes:

New opendata/ directory with the notebook
environment.yml: added earthkit-data>=1.0.0, boto3 (conda) and earthkit-plots>=1.0.0 (pip)
README: added an opendata/ entry and removed the outdated "frozen" note

Tested on Binder from this branch, builds and runs end-to-end: https://mybinder.org/v2/gh/valtzen/notebook-examples/add-heatwave-efi-notebook?labpath=opendata/heatwave-efi-2t-aws.ipynb

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[valtzen]

Hi @milanavuckovic
this is ready for review, I tested on Binder, runs clean.

[milanavuckovic]

Approved