## Context
More groundwork for the Explorer - this PR introduces the Markdown cells
and some basic (non data persisting) editing
Markdown cells will save either on
- Save button click
- Losing focus on the code editor
Hitting esc will cancel the changes
<img width="1387" height="674" alt="image"
src="https://github.com/user-attachments/assets/f0614b37-7a11-404f-9940-8bcd4de25c57"
/>
<img width="1087" height="516" alt="image"
src="https://github.com/user-attachments/assets/0a104168-2001-4ef7-934a-7e864956b3cb"
/>
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added editable Markdown cells to notebooks, with save and cancel
controls.
* Added drag-and-drop reordering for notebook cells, including keyboard
support.
* New notebooks now include sample Markdown content to help users get
started.
* **Improvements**
* Improved drag-handle placement and consistency across sortable
sections.
* Updated notebook empty and populated states to reflect the current
cell content.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Joshen Lim
b648db233b