Update contributing guidelines for images

[aneta-petrova]

In https://github.com/theforeman/foreman-documentation/blob/master/CONTRIBUTING.md, there is a section on how to contribute images to the documentation. Parts of it are obsolete so we should take a detailed look at the section and update it.

[aneta-petrova]

The image contribution guidelines should also mention the image-sources directory. See #4474 (review)