Add IMGPROXY_AVIF_SUBSAMPLE and avif_options docs

Author: DarthSim

docs/configuration/options.mdx

@@ -247,6 +247,10 @@ Cookies can contain sensitive information. Be careful when using `IMGPROXY_COOKI
 ### Advanced AVIF compression
 
 * [`IMGPROXY_AVIF_SPEED`]: controls the CPU effort spent improving compression. The lowest speed is at 0 and the fastest is at 9. Default: `8`
+* [`IMGPROXY_AVIF_SUBSAMPLE`]: ((pro)) controls when chroma subsampling is used. Supported values are:
+  * `auto`: _(default)_ use subsampling when the image is saved with quality less than `90`
+  * `on`: always use subsampling
+  * `off`: never use subsampling
 
 ### Advanced JPEG XL compression
 

docs/usage/processing.mdx

@@ -861,6 +861,15 @@ webpo:%compression:%smart_subsample:%preset
 
 Allows redefining WebP saving options. All arguments have the same meaning as with the [Advanced WebP compression](../configuration/options.mdx#advanced-webp-compression) configs. All arguments are optional and can be omitted.
 
+### AVIF options ((pro)) {#avif-options}
+
+```imgproxy_url_option
+avif_options:%subsample
+avifo:%subsample
+```
+
+Allows redefining AVIF saving options. All arguments have the same meaning as with the [Advanced AVIF compression](../configuration/options.mdx#advanced-avif-compression) configs. All arguments are optional and can be omitted.
+
 ### Format
 
 ```imgproxy_url_option