fix: add new site pages to nav (#245)

# Description

Two new documentation pages were added but aren't in the site nav so are
not discoverable. Adds them to the correct sidebar navigation
categories.

Tested locally:

![image](https://github.com/microsoft/retina/assets/2940321/a000f195-91fb-4f46-ab35-97f2d70475b5)

![image](https://github.com/microsoft/retina/assets/2940321/d7109fbe-e66b-4e74-88c2-230db9f7a4d8)

Signed-off-by: Evan Baker <rbtr@users.noreply.github.com>

.gitignore

@@ -32,3 +32,5 @@ output
 
 dist/
 bin/
+
+image-metadata-*.json

site/sidebars.js

@@ -29,6 +29,7 @@ const sidebars = {
         'installation/grafana',
         'installation/cli',
         'installation/config',
+        'installation/verify-signed-images'
       ],
     },
     {
@@ -80,7 +81,14 @@ const sidebars = {
         'troubleshooting/basic-metrics',
       ],
     },
-    'contributing/readme',
+    {
+      type: 'category',
+      label: 'Contributing',
+      items: [
+        'contributing/readme',
+        'contributing/developing',
+      ],
+    }
   ],
 
 };