docs(artifacts): add Backblaze B2 to external-files reference guide

[goanpeca]

Description

Adds Backblaze B2 to the existing reference-artifact docs in three surgical spots, mirroring how CoreWeave is already referenced on the same page.

File: models/artifacts/track-external-files.mdx

1. Intro list of common external storage solutions: adds "Backblaze B2 Cloud Storage" alongside CoreWeave, S3, GCS, Azure, and NFS.
2. S3-compatible interface Note: already mentions CoreWeave and MinIO; expanded to include B2 with a concrete AWS_S3_ENDPOINT_URL=https://s3.us-west-001.backblazeb2.com example.
3. Cloud-credentials table: new row pointing at the Backblaze B2 Application Keys documentation.

Scope notes

• 1 file changed, 3 insertions(+), 2 deletions(-).
• Localized versions (ja/fr/ko) deliberately not touched, consistent with how other recent provider additions on this page were merged. Translation maintainers can pick these up in a follow-up sync.
• No new pages, no nav changes; purely additive content on an existing page.

Companion SDK PR adding _is_backblaze_endpoint + boto3 user-agent tagging: wandb/wandb#11938

Testing

• Local build succeeds without errors (mint dev)
• Local link check succeeds without errors (mint broken-links)
• PR tests succeed

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates the “Track external files” docs to explicitly include Backblaze B2 as a supported external storage option (via S3-compatible APIs) and adds a credentials reference for it.

Changes:

• Add Backblaze B2 Cloud Storage to the list of common external storage solutions.
• Expand the S3-compatible interface note to include Backblaze B2 and provide an example endpoint URL.
• Add Backblaze B2 credentials documentation link to the provider table.

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