Add link to source code repository to about page

src/components/routes/About.vue

@@ -1,11 +1,13 @@
 <template lang="pug">
+- const REPO_LINK = "https://git.dietrich-rink.de/dietrich/vue-example-app"
 - const JOKE_API_LINK = "https://github.com/15Dkatz/official_joke_api"
 
 section
   h2 About
 
   p
-    | This is an example single page application built in Vue.js.
+    | This is an example single page application built in Vue.js. You can find
+    | its source code in #[a(href=REPO_LINK target="_blank") this repository].
   p
     | It uses nested components, routing, composables and the fancy language
     | extensions pug, typescript and sass, that compile to html, javascript and