Wrap section descriptions in <div>, not <p> (r-lib#2446)

Fixes r-lib#2352

NEWS.md

@@ -1,5 +1,6 @@
 # pkgdown (development version)
 
+* `build_article_index()` and `build_reference_index()` use an improved BS5 template that correctly wraps each section description in a `<div>`, rather than a `<p>`. This eliminates an empty pargraph tag that preceded each section description (#2352).
 * `build_news()` now warns if it doesn't find any version headings, suggesting that that `NEWS.md` is structured incorrectly (#2213).
 * `build_article()` gains a new `new_process` argument which allows to build a vignette in the current process for debugging purposes. We've also improved the error messages and tracebacks if an article fails to build, hopefully also making debugging easier (#2438).
 * Preview links now work once again (#2435).

inst/BS5/templates/content-article-index.html

@@ -8,7 +8,7 @@ <h1>{{{pagetitle}}}</h1>
     {{#sections}}
     <div class="section {{class}}">
       <h3>{{{title}}}</h3>
-      <p class="section-desc">{{{desc}}}</p>
+      <div class="section-desc">{{{desc}}}</div>
 
       <dl>
         {{#contents}}

inst/BS5/templates/content-reference-index.html

@@ -8,7 +8,7 @@ <h1>{{{pagetitle}}}</h1>
     {{#rows}}<div id="{{slug}}" class="section level2">
       {{#title}}<h2>{{{.}}}</h2>{{/title}}
       {{#subtitle}}<h3>{{{.}}}</h3>{{/subtitle}}
-      {{#desc}}<p class="section-desc">{{{desc}}}</p>{{/desc}}
+      {{#desc}}<div class="section-desc">{{{desc}}}</div>{{/desc}}
 
       {{#topics}}<dl>
         <dt>