docs: Add COSMIC and fix broken contribution link

CONTRIBUTING.md

@@ -1,4 +1,4 @@
 # XDG Desktop Portal - Contributing Guide
 
 The contribution guidelines have been moved to the online documentation:
-https://flatpak.github.io/xdg-desktop-portal/docs/contributing.html
+https://flatpak.github.io/xdg-desktop-portal/docs/for-contributors.html

doc/website/index.md

@@ -60,6 +60,7 @@ Here are some examples of available backends:
 - wlroots [xdg-desktop-portal-wlr](https://github.com/emersion/xdg-desktop-portal-wlr)
 - Deepin [xdg-desktop-portal-dde](https://github.com/linuxdeepin/xdg-desktop-portal-dde)
 - Xapp (Cinnamon, MATE, Xfce) [xdg-desktop-portal-xapp](https://github.com/linuxmint/xdg-desktop-portal-xapp)
+- COSMIC [xdg-desktop-portal-cosmic](https://github.com/pop-os/xdg-desktop-portal-cosmic)
 
 ## Design Considerations