Config: Add link to apps/Flathub home

Not 100% sure what the right copy is here but I've also been annoyed it's not simple to click to get back over to flathub.org.

Alternatively, we could make the logo itself link to `https://flathub.org`, and change the `Publish` link on the home page to point to `https://docs.flathub.org` or something, but I think this is fine to start.

Fixes #244

docusaurus.config.js

@@ -115,6 +115,11 @@ const config = {
             label: "Docs",
           },
           { to: "/blog", label: "Blog", position: "left" },
+          {
+            href: "https://flathub.org",
+            label: "Apps",
+            position: "right",
+          },
           {
             href: "https://github.com/flathub/documentation",
             label: "GitHub",