fix(spacing): connect section spacing to design token system

Wire .hbb-section to var(--hb-spacing-section) instead of hardcoded 6rem,
enabling hugoblox.layout.spacing (compact/comfortable/spacious) to take effect.

- Default to "spacious" (6rem) to preserve existing visual layouts
- Per-section override via design.spacing.padding still works (inline > class)
- Remove dead design.spacing frontmatter field from all templates
- Set correct layout.spacing token in each template's params.yaml

Closes #3314

templates/documentation/content/_index.md

@@ -3,10 +3,6 @@ title: 'Home'
 date: 2023-10-24
 type: landing
 
-design:
-  # Default section spacing
-  spacing: "6rem"
-
 sections:
   - block: hero
     content: