[feat] Add "Deploy to Render" button

[cybardev]

Adds:

• render.yaml Render Blueprint file
• Deploy to Render button in README

Thus people can conveniently deploy without their own fork, pulling the prebuilt image from upstream, with a single click^[1].

[1] Not really, since they'd still need to do some stuff over at Render, but this is much more streamlined than the instructions at https://docs.sidestore.io/docs/advanced/anisette/