test(section): add test for section nesting and level transitions
This MR adds a TCK test for Section block nesting and level transitions, demonstrating child section nesting (level 1 → level 2 → level 3) and returning to a sibling section at a lower numerical level (level 3 → level 1).
Added Test Files:
tests/block/section/nesting-config.jsontests/block/section/nesting-input.adoctests/block/section/nesting-output.json
Relates to asciidoc-lang#67