fix(docs): let the capability detail box grow to fit content on mobile

!519 (merged) fixed the side-by-side case (align-items: stretch), but on narrow viewports the diagram and capability panel stack instead of sitting in the same row — both on the docs landing page (sphinx-design grid collapses to one column below its md breakpoint) and on the embed page (its own flex-wrap kicks in earlier). Once stacked there's no taller sibling left for .capability-detail's flex: 1 1 0 to stretch to, so it collapses to its min-height and the hover/pinned description gets cut short and needs scrolling, even though there's plenty of room below it.

Add a max-width media query so .capability-detail sizes to its content instead of relying on stretch once things stack. landing.css is shared unmodified by both pages (the embed page links it directly, see conf.py), so this fixes the embed page too — verified by loading embed/architecture.html directly at a 390px viewport and confirming the description box's height now matches its content instead of being clipped.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com