help: Ensure trailingSlash is set to never.

Sidebar links were adding a trailing slash to each of the pages. Our
canonical URLs don't have a trailing slash. This will probably also stop
starlight from adding trailing slash at other unexpected places.

When running the astro dev server, if a page has a trailing slash, we
will get a page from astro asking to remove that slash. That will not
happen in static build mode or production since we don't use astro's
serving mechanisms there.

starlight_help/astro.config.mjs

@@ -10,6 +10,7 @@ import Icons from "unplugin-icons/vite";
 // https://astro.build/config
 export default defineConfig({
     base: "help",
+    trailingSlash: "never",
     vite: {
         plugins: [
             // eslint-disable-next-line new-cap