docs: add migration guide with every breaking (major) release

[NoopDog]

Summary

Each major version release of findable-ui should include a migration guide documenting breaking changes and how consuming apps need to update.

Motivation

Consuming apps (ncpi-dataset-catalog, anvil-portal, etc.) need to upgrade findable-ui regularly. Currently, the only reference for breaking changes is the CHANGELOG.md (generated by release-please), which lists commit titles but not the steps needed to migrate. This makes upgrades time-consuming as developers have to read through diffs to figure out what changed and how to adapt.

Proposal

• Include a migration guide section in each major release (e.g., MIGRATION.md or a section in the release notes)
• For each breaking change, document:
  • What changed
  • Why it changed
  • Before/after code examples
  • Any new dependencies or config changes required
• Consider a docs/migration/ directory with per-version guides (e.g., v50.md, v51.md)

Acceptance Criteria

• Agree on format and location for migration guides
• Add migration guide for the most recent major release as a starting point
• Document the expectation in CONTRIBUTING.md or similar so future breaking PRs include migration notes