docs(bedrock): Bedrock fine-tuning reference + PT/Reserved Tier guidance

[rsmets]

Summary

Adds a field-tested Bedrock fine-tuning reference and aligns the capacity-reservation guidance across the aws-dev-toolkit Bedrock skill (SKILL.md, references/fine-tuning.md, references/cost-modeling.md).

The core of the customization guidance — when to reach for Provisioned Throughput (PT) vs. Reserved Tier vs. on-demand custom deployment — was refined per feedback from Justin Muller on the Bedrock service team, shared on the internal SA Slack thread about PT governance and its successor.

What Justin's input changed

• PT vs. Reserved Tier are different infrastructures, not interchangeable options. PT reserves capacity on traditional Bedrock; Reserved Tier reserves capacity on Mantle. A given model is usually on one or the other — so it's a per-model availability question.
• Lean toward Reserved Tier where a model offers it (better UX/capabilities), and fall back to PT when RT isn't available for that model.
• Bring Your Own Model (BYOM) is the Mantle successor to Custom Model Import — added as a custom-model path.
• No hard "PT is deprecated" claim. There's no published deprecation notice, so the docs avoid asserting one; they recommend on-demand custom deployment first and verifying the per-model matrix against current AWS docs.

Files

• plugins/aws-dev-toolkit/skills/bedrock/references/fine-tuning.md — fine-tuning reference + PT/RT/BYOM framing
• plugins/aws-dev-toolkit/skills/bedrock/references/cost-modeling.md — PT/Reserved Tier capacity-reservation entries
• plugins/aws-dev-toolkit/skills/bedrock/SKILL.md — gotcha pointers updated to match

Note on public publication

This plugin ships publicly (outside AWS), so the content is kept to publicly-documented facts. Mantle, Reserved Tier, and BYOM are public/launched; the only thing intentionally held back is unofficial deprecation speculation.

🙏 Thanks to Justin Muller (Bedrock service team) for the clarifications that shaped this guidance.