update nav dropdown behavior (#23436)

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## Description

Change how the section drop-down arrows are rendered in the side-nav.

Currently, they generate for any folder, even if it just contains an
index. To not generate the arrow, the index would have to be taken out
of the folder and renamed as the folder.

This change now counts how many children under the index, and only
generates the arrow if it has children (that aren't hidden from the
nav).

For example, see how there is an arrow for the mcp gateway, but it
expands to nothing.
Current: https://docs.docker.com/ai/mcp-gateway/
Updated:
https://deploy-preview-23436--docsdocker.netlify.app/ai/mcp-gateway/

## Related issues or tickets

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## Reviews

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- [ ] Editorial review

Signed-off-by: Craig <craig.osterhout@docker.com>

layouts/partials/sidebar/sections.html

@@ -79,6 +79,9 @@
           </button>
         {{- end }}
       </div>
+      {{/* Only show expand group button if section has children */}}
+      {{- $hasChildren := gt (len (where .Pages "Params.sitemap" "ne" "false")) 0 }}
+      {{- if $hasChildren }}
       <!-- Expand group button -->
       <button
         @click="expanded = !expanded"
@@ -97,6 +100,7 @@
           {{ partialCached "icon" "arrow_drop_up" "arrow_drop_up" }}
         </span>
       </button>
+      {{- end }}
     </div>
     <ul
       :class="{ 'hidden' : !expanded }"