docs: Add favicons

See https://gitlab.gnome.org/GNOME/gi-docgen/-/merge_requests/201.
Generated by
https://gitlab.gnome.org/World/design/emblem/-/merge_requests/40.

The icon used was portal-symbolic from the devkit, using colors #26A269
to #33D17A were taken from the source of the SVG.

Author: A6GibKm

doc/favicon-128x128.png

@@ -41,4 +41,8 @@ content_files = [
 ]
 content_images = [
   "libportal.svg",
+  "favicon-32x32.png",
+  "favicon-128x128.png",
+  "favicon-180x180.png",
+  "favicon-192x192.png",
 ]