seo: add tag pages to sitemap

src/pages/sitemap.xml.ts

@@ -11,6 +11,14 @@ export async function GET(context: APIContext) {
       <url><loc>${context.site}services</loc></url>
       <url><loc>${context.site}cinema</loc></url>`
 
+  notes
+    .map(entry => entry.data.tags)
+    .flat()
+    .filter((tag, i, self) => self.findIndex(t => t.slug === tag.slug) === i)
+    .forEach(tag => {
+      sitemap += `<url><loc>${context.site}tags/${tag.slug}</loc></url>`
+    })
+
   notes.forEach(note => {
     sitemap += `<url><loc>${context.site}notes/${note.slug}</loc></url>`
   })