Unavailable Yoruba flag

For me it looks like Yoruba flag is also unavailable from [this link](https://www.fotw.info/images/n/ng%7Deoyor.gif). Since it's not some official url, maybe we can consider moving it to the [assets](https://github.com/firstcontributions/assets) project.

Here is the image from my cache:

Start by checking the linked FOTW image and the linked assets project to understand where the Yoruba flag should live; the issue names no repository file or test. Done means the Yoruba flag is available from a project-controlled asset location instead of only the unavailable external URL.

Written by the indexing model from the issue text.