[11.x] Improve File validation structure and clarify File::image() usage

[SanderMuller]

Split from #10136

This PR enhances the validation documentation with the following updates:

1. Improved Structure:

   • Adds consistent headings for file validation rules like file sizes and image dimensions.
   • Separates the explanation of dimensions from File::image() for better focus.

2. Expanded File::image() Details:

   • Clearly lists supported file types (jpg, jpeg, png, bmp, gif, svg, webp).
   • Adds a warning about potential XSS risks with SVG files and how to handle them safely.

3. Streamlined Examples:

   • Moves method chaining examples (e.g., dimensions) to dedicated sections for clarity.
   • Includes a concise example for file size validation.