docs: restructure README and adoption guides around contract lifecycle architecture

Reframed documentation to present the project as a reference architecture
for contract-driven, generics-aware API boundaries.

Clarified server/client lifecycle responsibilities, made generic support scope explicit,
and simplified client build guidance around deterministic model rendering governance.

Improves conceptual flow, terminology consistency, and long-term maintainability.
No runtime changes.

Author: bsayli

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -39,7 +39,7 @@ body:
       description: "Tool versions or environment details."
       placeholder: |
         - Java: 21
-        - OpenAPI Generator: 7.20.0
+        - OpenAPI Generator: 7.21.0
         - Spring Boot: 3.5.x
         - OS: macOS / Windows / Linux
   - type: textarea