Add section about the Image URL Generator (6.0)

[lb-]

Wagtail 6.0 has some great improvements to the Image URL Generator that would be good to highlight in the what's new part for this release.

Additionally it would be good to explain this page on the guide as it's often forgotten and can be quite helpful to editors.

I suggest we just add a new section on this page https://guide.wagtail.org/en-latest/how-to-guides/manage-images/

See

• https://github.com/wagtail/wagtail/pull/11317
• https://github.com/wagtail/wagtail/pull/11181
• https://github.com/wagtail/wagtail/issues/3683