Document ARKINDEX_MAX_IMAGE_PIXELS

https://redmine.teklia.com/issues/8024

tasks#195 (closed) adds support for ARKINDEX_MAX_IMAGE_PIXELS, which is intended to be set in the backend's configuration so it is sent along to all processes. Please document it as ponos.default_env.ARKINDEX_MAX_IMAGE_PIXELS.

The docs should link to Pillow's documentation on MAX_IMAGE_PIXELS, and mention that this only impacts file imports, but this variable may also be used by any worker that needs to support very large images.