feat: subtype tree documentation and release notes

[mkirk111]

This pull request updates the PeCan documentation to introduce and describe the new Subtype Tree feature, which enhances navigation and exploration of pediatric cancer diagnoses using the CC4K framework. The changes also improve the clarity and structure of the Diagnosis Search section and update the release notes to reflect the latest features.

New Subtype Tree Feature and Documentation Updates:

• Added a new section in the documentation describing the Subtype Tree, which provides a hierarchical, navigable view of pediatric cancer diagnoses, allows node expansion to reveal subtypes, displays sample counts, includes node tooltips, and supports SVG download.
• Updated the Diagnosis Search section to clarify the distinction and relationship between the Sunburst and Subtype Tree tools, and improved explanations of their navigation and data exploration capabilities.

Release Notes Enhancements:

• Added a May 2026 entry to the release notes, highlighting the introduction of the Subtype Tree, node tooltips, and download options for both the tree and metadata.