Consolidate migration docs and create 0.x to 1.0 migration guide

- Create comprehensive migration_guide_0x_to_1x.md covering model-level indexing and datetime field changes
- Remove separate migration docs (datetime_schema_detection.md, MIGRATION_GUIDE.md, MIGRATION_PERFORMANCE_TUNING.md, MIGRATION_TROUBLESHOOTING.md)
- Update migrations.md to reference new guide and include essential troubleshooting
- Add migration guide references to index.md and getting_started.md

Author: abrookins