chore: set up oranda

Sets up oranda so we can get nice website things for free. Some caveats we have right now:

- Absolutely manual, I have to do `oranda build` and the deploy manually right now
- I had to pop the Readme into a new Markdown file because the first header in there was looking very strange

oranda.json

@@ -0,0 +1,11 @@
+{
+  "homepage": "https://rustlings.cool",
+  "readme_path": "website.md",
+  "repository": "https://github.com/rust-lang/rustlings",
+  "analytics": {
+    "plausible": {
+      "domain": "rustlings.cool"
+    }
+  },
+  "changelog": true
+}