Complete CHANGELOG for r2.3 release

[XunliYang]

Summary

• Complete CHANGELOG-r2.md with release details for r2.3
• Update version descriptions for network-slice-booking 0.2.0 and network-slice-assignment 0.1.0
• Fill in Added/Changed/Removed sections with actual PR references

Changes

• network-slice-booking 0.2.0: Added alignment with Commonalities r4.3, schema fixes, and CloudEvent $refs
• network-slice-assignment 0.1.0: Same updates for consistency

Related PRs

• Align with Commonalities r4.3 - Add mandatory templates and fix schemas #130: Align with Commonalities r4.3
• Update API definitions with corrected defaults and schema constraints-align with commonalities r4.2 #116: Update API definitions with corrected defaults
• Fix validation issues: Adopt Commonalities r4.2 $refs for common event schemas  #120: Replace inline schemas with $refs