[12.x] Document image rule SVG security changes

[SanderMuller]

This pull request updates the validation documentation to reflect the 12.x changes for image validation, focusing on secure defaults and better guidance for developers.

Key Updates:

• SVG Default Exclusion:
  The documentation highlights that SVG files are excluded by default in the image and File::image() rules due to potential XSS risks.

• Opt-In SVG Allowance:
  Provides examples for explicitly allowing SVGs using image:allow_svg or File::image(allowSvg: true). Developers are encouraged to sanitize SVGs manually if they enable this option.

• Actionable Warnings:
  Adds warning blocks to raise developer awareness about SVG risks and encourage secure handling practices.

[taylorotwell]

Can you undo all of the unrelated changes to the XSS stuff and resubmit please? It makes this hard to follow and those changes can be submitted separately to 11.x.

[SanderMuller]

Can you undo all of the unrelated changes to the XSS stuff and resubmit please? It makes this hard to follow and those changes can be submitted separately to 11.x.

I think I've scoped it down to what you've asked for