Stop uploading notebook test results to downloads.rapids.ai

Description

This project currently runs a bunch of RAPIDS notebooks, collects their logs in a few tarballs, and uploads those tarballs to downloads.rapids.ai.

See paths like the following at downloads.rapids.ai:

ci/docker/pull-request/{pr_number}/{commit_sha}/test_notebooks_output_*.tar.gz

That comes from here:

https://github.com/rapidsai/docker/blob/bebddd8d3ef10d19773bc92607e825c727e0c9b0/.github/workflows/test-notebooks.yml#L108

Those uploads should either be removed (if they are determined to be unused) or switched to using the GitHub Actions artifact store.

Benefits of this work

• another step towards removing downloads.rapids.ai completely
• would allow contributors who don't work at NVIDIA to access those test result outputs from CI

Acceptance Criteria

• this repo does not depend on downloads.rapids.ai in any way
• docs accurately reflect artifact-handling in this repo

Approach

Try using actions/upload-artifact in test workflows, similar to this:

https://github.com/rapidsai/shared-workflows/blob/d87850d9ccd0c6b1c610a0bf533e1acf1731f844/.github/workflows/wheels-build.yaml#L272-L277

Notes

N/A

Start with .github/workflows/test-notebooks.yml around line 108 and search this repository for downloads.rapids.ai references. Compare the notebook workflow with the shared-workflows wheels-build.yaml artifact example. Done means the repository no longer depends on downloads.rapids.ai and its documentation accurately describes artifact handling.