PB-1877 Set base path for test links

For test links, we need to take into account that there is a "/preview/feat-PB-1234-my-branch" in between. If we don't add this to the static asset links, it will result in broken styling, missing images and broken page links.

See ["Base URL"](https://vitepress.dev/guide/asset-handling#base-url) in the VitePress docs.

Author: asteiner-swisstopo

.vitepress/config.mts

@@ -5,6 +5,16 @@ import fs from "fs";
 export default defineConfig({
   title: "*.geo.admin.ch",
   description: "Technical Documentation about *.geo.admin.ch",
+  /*
+  Prepend a base path such that links still work when the page is built for the
+  preview in a PR ("test link" in the PR description).
+
+  For example, VITE_BASE_URL would be set to "/preview/feat-pb-1234-my-branch/"
+  for a link like this:
+
+     https://sys-docs.dev.bgdi.ch/preview/feat-pb-1234-my-branch/index.html
+  */
+  base: process.env.VITE_BASE_URL || '/',
   head: [["link", { rel: "icon", href: "/favicon.ico" }]],
   ignoreDeadLinks: "localhostLinks",
   vite: {