Add OSCAL 800-171A Rev.2 Assessment Plan and Generated Catalog (Example)

[khanssen]

This pull request adds a validated example of an OSCAL assessment plan and a generated full catalog derived from NIST SP 800-171 Rev. 2 and SP 800-171A Rev. 2 for community reference, validation, and educational use.
This PR uses a fully expanded catalog that includes control statements, objectives, and metadata to support resolvability for related-controls and end-to-end schema validation.
The assessment plan references these controls by ID only, so an official catalog update or replacement can be dropped in later without changing any IDs or plan content.

Summary of changes

Adds examples/assessment-plan/json/nist-sp-800-171a-rev2-assessment-plan.wired.json, a structured OSCAL assessment plan using consistent related-control-id references.
Adds examples/catalog/json/nist-sp-800-171-rev2-catalog.generated.json, a fully expanded OSCAL catalog derived from NIST SP 800-171 Rev. 2.
Adds examples/README.md, documentation explaining validation steps, linkage, and file relationships.

Validation
Conforms to OSCAL 1.1.2 and passes schema validation with no unresolved control references:
bashoscal validate --no-schematron examples/catalog/json/nist-sp-800-171-rev2-catalog.generated.json
oscal validate --no-schematron examples/assessment-plan/json/nist-sp-800-171a-rev2-assessment-plan.wired.json

Notes
These files are community-derived examples and not official NIST publications. They are provided under the CC0 1.0 Universal Public Domain Dedication.
Checklist
✅ Correct base branch selected per Contributing guidance
✅ "Allow edits and access to secrets by maintainers" enabled
✅ No duplicate pull requests exist for this content
✅ Commits squashed and CI/CD checks expected to pass
✅ Examples and supporting documentation included

[khanssen]

all comments reviewed and resolved.