Add docs for the images context API

[timleslie]

Add docs for the images context API and makes some cosmetic changes to the images context implementation.

[changeset-bot]

🦋 Changeset detected

Latest commit: af8ae8a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@keystone-next/keystone	Patch
@keystone-next/test-utils-legacy	Patch

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[gwyneplaine]

lgtm