Some small tidying up of the docs site

Signed-off-by: Peter Macdonald <peterm4c@pm.me>

microsite/docusaurus.config.ts

@@ -273,6 +273,10 @@ const config: Config = {
             from: '/docs/getting-started/app-custom-theme',
             to: '/docs/conf/user-interface',
           },
+          {
+            from: '/docs/plugins/existing-plugins',
+            to: '/docs/plugins/',
+          },
         ],
       }),
     [

microsite/sidebars.ts

@@ -397,7 +397,6 @@ export default {
       },
       [
         'plugins/index',
-        'plugins/existing-plugins',
         'plugins/create-a-plugin',
         'plugins/plugin-development',
         'plugins/structure-of-a-plugin',