Add JSON type support for SQLAlchemy dialect

Implement JSON type support to resolve GitHub Issue #619.
This enables proper handling of JSON columns when reflecting
Iceberg tables and querying JSON data with SQLAlchemy.

Changes:
- Add visit_JSON method to AthenaTypeCompiler
- Update ischema_names mapping from String to JSON type
- Add unit test for JSON type compilation
- Add integration test for JSON type with CAST operations
- Update documentation with JSON Type Support section
- Reorganize documentation structure (move Query Execution Callback section)

Limitations:
- Athena only supports JSON objects, not top-level arrays
- JSON type is for DML operations (SELECT), not DDL (CREATE TABLE)

Fixes #619

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: laughingman7743