Image scaling developer docs

What does this MR do?

Relates to #268242 (closed)

This adds developer documentation for image scaling, which can be difficult to grok without guidelines.

Since this didn't quite fit in any existing category, I added it as a Backend guide in the Contributor section.

Preview: http://docs-preview-ee-47506.178.62.207.141.nip.io/ee/development/image_scaling.html

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content