v2.1.7: Update changelog, demo, and core for Version 2.1.7

- Added `Version 2.1.7` section to `changelog.md` with feature, improvement, and bugfix notes.
- Introduced `overlap` support in day/week views for stacked appointment rendering.
- Enhanced overlap stacking order for better drag-move behavior.
- Updated demo with overlap examples and toggle in appointment modal.
- Refreshed documentation to cover the new `overlap` option and associated changes.