docs: clarify Trainer runner-count tuning

[Jimmy Wang (JimmyWang0417)]

Summary

Adds a runner-count tuning section to the parallelization tutorial for issue #366. It explains:

• how n_runners maps to rollout concurrency and how the deprecated n_workers alias relates to it;
• why runner count is separate from store and proxy worker counts;
• a practical 1/2/4 sweep and the throughput, latency, rate-limit, CPU, memory, and GPU KV-cache trade-offs;
• why n_runners=1 is useful for debugging and a baseline.

The guidance avoids claiming a universal optimum or inventing benchmark numbers.

Fixes #366

Validation

• mkdocs build --strict
• PRE_COMMIT_HOME=/tmp/agent-lightning-366-precommit pre-commit run --files docs/tutorials/parallelize.md --show-diff-on-failure
• git diff --check

AI assistance was used to prepare this patch. I reviewed the changed lines and ran the checks above.

[Jimmy Wang (JimmyWang0417)]

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