Document serverless W&B run-finished alert behavior

[poofeth]

Summary

• document that each ServerlessBackend.train(...) call submits a separate remote training job with W&B-backed artifacts/runs
• call out why W&B "Run finished" Slack alerts can fire once per training step
• add a cross-link from Tracking Metrics to the ServerlessBackend lifecycle note

Fixes #638

Validation

• rg -n "Run finished|backend\.train|one notification|serverless training lifecycle|W&B-backed artifacts" docs/fundamentals/art-backend.mdx docs/features/tracking-metrics.mdx
• git diff --check

Note: uv run ruff check/format was not applicable because these MDX docs are parsed as Python by Ruff in this repo.

[poofeth]

Validation evidence for this docs-only fix:

• Confirmed the new docs text with rg -n "Run finished|backend\.train|one notification|serverless training lifecycle|W&B-backed artifacts" docs/fundamentals/art-backend.mdx docs/features/tracking-metrics.mdx.
• Ran git diff --check; no whitespace issues.
• uv run ruff check/format is not applicable to these MDX docs because Ruff parses them as Python in this repo.

[poofeth]

Closing this non-bounty / low-engagement PR to keep my open review queue focused on active bounty and maintainer-requested work.