feat: add Render Blueprint for one-click self-host (#15478)

Adds one-click deploy to [Render](https://render.com) for self-hosters.
The change is additive: a root `render.yaml`, two small files under
`.render/`, and one button in the existing Self-Host section of the
README.

.render/start.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -eu
+
+# Use the hostname Render assigns, unless AFFINE_SERVER_HOST is already set to a
+# custom domain.
+export AFFINE_SERVER_HOST="${AFFINE_SERVER_HOST:-${RENDER_EXTERNAL_HOSTNAME:-localhost}}"
+
+# Generates the server private key on first boot, then runs the Prisma and data
+# migrations. Safe to run on every boot.
+node ./scripts/self-host-predeploy.js
+
+exec node ./dist/main.js