refactor: improve item tree display #1853

Job	Run time
Check	1m 18s
Test mdbook	44s
Check Style	1m 20s
Doctest Rust (ubuntu-latest)	1m 34s
Test Rust (ubuntu-latest)	4m 2s
Doctest Rust (windows-latest)	2m 57s
Test C++ Runtime (ubuntu-latest)	3m 35s
Doctest Rust (macOS-latest)	3m 2s
Test Rust (windows-latest)	9m 39s
Test C++ Runtime (windows-latest)	7m 42s
Test C++ Runtime (macOS-latest)	5m 29s
Test Rust (macOS-latest)	6m 40s
	48m 2s

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