Prepare release

Author: justinmayer

RELEASE.md

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+Release type: minor
+
+* *Feature:* Add `IMAGE_PROCESS_ADD_CLASS` and `IMAGE_PROCESS_CLASS_PREFIX` settings to control
+the `image-process-<transform>` CSS class assignment for processed images.
+* *Fix*: Encode URLs in `srcset` when they contain a space or a comma, as those characters have a special meaning in the context of a `srcset` value.
+* *Fix:* Improve operations for images with empty margins by using `image.width`/`image.height` instead of `bbox`.
+* *Fix:* Preserve HTML indentation when rewriting content.
+* *Fix:* Avoid loading images needlessly by only processing images if Pillow can handle them.
+* *Tests:* Make tests more resilient to small pixel content differences on ARM vs. Intel architectures.
+* *Docs:* Correct cropping transformation values.