fix(site): restore .prose class on docs and tutorial bodies (#132)

The Astro migration (b8f09c8) replaced the old generator's
`<section class="skill-detail-editorial prose">` wrapper with
`<div class="skills-detail-body docs-body">`, dropping the prose
class. The .prose rules in sub-pages.css were left intact but no
longer applied, so markdown bodies fell back to default browser
margins — heading top-margin shrank from 2.2em to ~0.83em and
line-height from 1.7 to 1.6, which read as cramped vertical
rhythm on mobile.

Co-authored-by: Claude <noreply@anthropic.com>

site/layouts/Doc.astro

@@ -76,7 +76,7 @@ for (const cat of CATEGORY_ORDER) {
           {tagline && <p class="sub-page-lede">{tagline}</p>}
         </header>
 
-        <div class="skills-detail-body docs-body">
+        <div class="skills-detail-body docs-body prose">
           <slot />
         </div>
 

site/pages/tutorials/[...slug].astro

@@ -37,7 +37,7 @@ const { Content } = await render(entry);
       {entry.data.tagline && <p class="sub-page-lede">{entry.data.tagline}</p>}
     </header>
 
-    <div class="skills-detail-body docs-body tutorial-body">
+    <div class="skills-detail-body docs-body tutorial-body prose">
       <Content />
     </div>
   </div>