The first link under a parent (e.g. the first submenu article) had no gap
above it while every other menu item had a uniform 16px gap. Inter-item
spacing was incidentally coming from the global `ul li { margin-bottom }`
rule in wordpress-blocks.css; a submenu's first item never got it because
`.soames-docs-menu { margin: 0 }` zeroes the submenu's top margin.

Give the docs menu its own vertical rhythm: reset the leaked global margin,
add a uniform step between siblings, and a matching step above each submenu.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>