Only include titles in summaries (#1549)

I previously made this change for HTTP endpoints in #1446. It seems there are
other places where we have over-sized `<summary>` elements. I would like
to elimintate those too, as grumbled about in
https://github.com/matrix-org/matrix-spec/issues/1358#issuecomment-1325220859
and
https://github.com/matrix-org/matrix-spec/issues/1352#issuecomment-1439033334

layouts/shortcodes/definition.html

@@ -39,6 +39,8 @@
  <code>{{ $definition.title }}</code>
 </h1>
 
+</summary>
+
 <hr/>
 
 {{ if (index $definition "x-addedInMatrixVersion") }}
@@ -47,7 +49,6 @@
 
 {{ $definition.description | markdownify }}
 
-</summary>
 
 {{ $additional_types := partial "json-schema/resolve-additional-types" (dict "schema" $definition "name" (printf "\"%s\"" $path)) }}
 {{ $additional_types = uniq $additional_types }}

layouts/shortcodes/event-fields.html

@@ -26,12 +26,12 @@
 <h1>
  <code>{{ humanize $event.title }}</code>
 </h1>
+</summary>
 
 <hr/>
 
 {{ $event.description | markdownify }}
 
-</summary>
 
 {{ $event = merge $event (dict "title" "") }}