Explain image formats

[lb-]

Now that Wagtail has added support for a wider range of image formats in recent releases it may be good to provide a user guide overview of the various formats.

Some users may not really understand what they're uploading and also Wagtail makes it easier to render a different output. This may not be critical but I thought I would suggest it.

See

• https://docs.wagtail.org/en/latest/releases/5.1.html#avif-image-support
• https://docs.wagtail.org/en/latest/releases/2.7.html#webp-image-support
• https://docs.wagtail.org/en/latest/releases/5.0.html#svg-image-support