Add deploy.sh + .deploy.env.example; update index.html (sponsor checkout link)

.deploy.env.example

@@ -0,0 +1,7 @@
+# Deploy credentials for inference.coop Surfer (WebDAV).
+# This file is gitignored — never commit it.
+# Copy to .deploy.env and fill in the values.
+
+SURFER_URL=https://inference.coop/_webdav/
+SURFER_USER=
+SURFER_TOKEN=