avifenc: add page (#7636)

pages/linux/avifenc.md

@@ -0,0 +1,12 @@
+# avifenc
+
+> AV1 Image File Format (AVIF) encoder.
+> More information: <https://aomediacodec.github.io/av1-avif/>.
+
+- Convert a specific PNG image to AVIF:
+
+`avifenc {{path/to/image.png}} {{path/to/image.avif}}`
+
+- Encode with a specific speed, where 0=slowest, 10=fastest, and 6=default:
+
+`avifenc --speed {{2}} {{path/to/image.png}} {{path/to/image.avif}}`