Problem:
Treemd currently shows the full heading tree, regardless of whether sections contain actionable items. This makes it harder to focus on parts of a document that still have open work, especially in large Markdown files with many completed sections.

Proposal:
Add an option to filter the existing heading tree so that only headings which contain open to-dos "- [ ]" are shown. A heading should remain visible if it either directly contains an open to-do in its content or if any of its child headings contain open to-dos. The original heading hierarchy and tree structure should remain intact.

Benefits:
Improved focus on actionable sections without losing document context.
No need for a separate to-do tree; the existing heading hierarchy can be reused.
Lower implementation complexity compared to parsing and rendering a full task hierarchy.
Better usability for task-driven workflows in Markdown files.