[ci] Add pointer to template image assets wiki (#103787)

Many of our flutter_tools templates include image assets such as
application icons. Adding these will trigger an analysis error from our
CI tooling.

This adds a pointer to the wiki describing the procedure for adding or
updating image assets in the templates:
https://github.com/flutter/flutter/wiki/Managing-template-image-assets

Uncovered while working on https://github.com/flutter/flutter/issues/103371

dev/bots/analyze.dart

@@ -1420,6 +1420,8 @@ Future<void> verifyNoBinaries(String workingDirectory, { Set<Hash256>? legacyBin
         'size of the repository as it is distributed to all our developers. If you have a binary',
         'to which you need access, you should consider how to fetch it from another repository;',
         'for example, the "assets-for-api-docs" repository is used for images in API docs.',
+        'To add assets to flutter_tools templates, see the instructions in the wiki:',
+        'https://github.com/flutter/flutter/wiki/Managing-template-image-assets',
       ]);
     }
   }

dev/bots/test/analyze_test.dart

@@ -167,6 +167,8 @@ void main() {
         'size of the repository as it is distributed to all our developers. If you have a binary\n'
         'to which you need access, you should consider how to fetch it from another repository;\n'
         'for example, the "assets-for-api-docs" repository is used for images in API docs.\n'
+        'To add assets to flutter_tools templates, see the instructions in the wiki:\n'
+        'https://github.com/flutter/flutter/wiki/Managing-template-image-assets\n'
         '━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━\n'
       );
     }