Document all four adapter registration locations

[abhinav-bellapu]

Description

Closes #1511.

The adapter creation guide listed three registration locations while the formal adapter specification listed only two. Both disagreed with the existing four-place invariant in contributing.md and supported_architectures/AGENTS.md.

This change:

• documents tools/model_registry/generate_report.py and ARCHITECTURE_DESCRIPTIONS as the fourth registration location;
• adds the two missing model-registry locations to the adapter specification; and
• updates the guide's final checklist and common-pitfall reminder to cover all four locations.

This keeps contributor instructions consistent and reduces the chance that a new adapter loads successfully but has incomplete discovery or generated reporting. No new dependencies are required.

Validation

• uv run --locked --python 3.12 build-docs (succeeded; no warnings from the changed files)
• make check-format
• uv run --locked --python 3.12 mypy .

Type of change

• This change requires a documentation update

Checklist

• I have made corresponding changes to the documentation
• My changes generate no new warnings
• No code behavior or interfaces were changed
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

[abhinav-bellapu]

@jlarson4 ready for review