Add a post about marking invoices as paid (#53)

Co-authored-by: Gary Tou <[email protected]>

content/posts/manually-mark-as-paid/post.mdx

@@ -0,0 +1,15 @@
+import post from '@/components/Post';
+export default post({
+  title: "🧾 Mark invoices as paid",
+  slug: "manually-mark-as-paid",
+  category: "new",
+  tags: [],
+  authors: ["sam"],
+  date: new Date("2025-01-26T16:00:00Z")
+});
+
+Ever had a sponsor pay an invoice by check? It's awkward because then you're stuck with an "unpaid" invoice on HCB. 
+
+You can now mark invoices as paid if a sponsor pays using alternative means (check, cash, etc.):
+
+<img src="https://cloud-j74awbtdh-hack-club-bot.vercel.app/00screenshot_2025-01-27_at_12.40.17___am.png" width="300px" />