docs: expand MIGRATION.md to CNA depth

[ulises-jeremias]

Summary

• Expand docs/MIGRATION.md to match the depth of the CNA migration guide, adapted for CPA and cpa-templates.
• Cover one-time snapshot semantics, uv/Dependabot updates, diff and selective re-scaffolding workflows, CNA-to-CPA renames, template URL/slug formats, CLI upgrades, adding extensions post-hoc, Cookiecutter/Copier migration, and help links.

Closes#170

Test plan

• Doc-only change; reviewed against CNA docs/MIGRATION.md for parity
• Verified CPA env vars and CLI flags against source code
• ASCII-only content

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