Add IMGPROXY_PDF_NO_BACKGROUND docs

Author: DarthSim

docs/configuration/options.mdx

@@ -660,6 +660,7 @@ We highly recommended reading the [Memory usage tweaks](../memory_usage_tweaks.m
 * [`IMGPROXY_ARGUMENTS_SEPARATOR`]: a string that will be used as a processing/info options arguments' separator. Default: `:`
 * [`IMGPROXY_USE_LINEAR_COLORSPACE`]: when `true`, imgproxy will process images in linear colorspace. This will slow down processing. Note that images won't be fully processed in linear colorspace while shrink-on-load is enabled (see below).
 * [`IMGPROXY_DISABLE_SHRINK_ON_LOAD`]: when `true`, disables shrink-on-load for JPEGs and WebP files. Allows processing the entire image in linear colorspace but dramatically slows down resizing and increases memory usage when working with large images.
+* [`IMGPROXY_PDF_NO_BACKGROUND`]: ((pro)) when `true`, imgproxy will not add a white background to the PDF pages. Default: `false`
 * [`IMGPROXY_STRIP_METADATA`]: when `true`, imgproxy will strip the output images' metadata (EXIF, IPTC, etc.). Default: `true`
 * [`IMGPROXY_KEEP_COPYRIGHT`]: when `true`, imgproxy will not remove copyright info while stripping metadata. Default: `true`
 * [`IMGPROXY_STRIP_METADATA_DPI`]: ((pro)) the DPI metadata value that should be set for the image when its metadata is stripped. Default: `72.0`