docs: add ZopDay to the deploy options - #1243

Adds one-click deployment with ZopDay to the Deployment row, alongside the Heroku and Koyeb
buttons. README only, +1/−0.

Disclosure up front: I work on ZopDay. Interstellar is AGPL-3.0 and the row already carries two
providers, so this is offered as an addition rather than a replacement for either.

ZopDay's One-Click Deploy feature lets users quickly deploy an application to ZopCloud or their
own cloud via ZopDay, without the usual manual deployment setup.

The feature supports applications from GitHub repositories and Docker images, while Helm Charts
can be deployed to the user's own cloud Kubernetes cluster.

Supported deployments

• GitHub Repositories → ZopCloud / Own Cloud via ZopDay
• Docker Images → ZopCloud / Own Cloud via ZopDay
• Helm Charts → Own Cloud Kubernetes clusters only

Users open a shareable deploy link and deploy with one click. The link can also carry preset
environment variables so required configuration fills in automatically. Try it out:
https://zop.dev/zopday/app/deploy

One-Click Deploy with ZopDay: https://zop.dev/zopday/oss

What changed

A third button in the existing row, in the same raw <a><img> shape as the Heroku and Koyeb
entries, placed after Koyeb.

What it offers

The button builds this repository and runs your existing npm start — node src/server.js —
on port 8080, which is what src/server.js already falls back to (process.env.PORT || 8080).
Nothing new to maintain and no extra entry point.

No environment variables are preset, so a deploy from this link behaves exactly like npm start:
config.js keeps challenge: false, and anyone who wants the password protection your
Password Protection section documents sets it themselves, the same way they would locally.

How it differs from the existing two

• Heroku deploys through your app.json on the heroku/nodejs image, and Koyeb builds
  from git on their own platform. Both are the right default for a quick personal instance.
• ZopDay can instead deploy into an AWS, GCP or Azure account the user already owns, so the
  instance and its traffic stay inside their own cloud account and billing rather than on a
  shared platform.
• ZopDay also supports Helm Charts into a Kubernetes cluster the user already runs.

So it complements the two entries already here rather than duplicating either of them.

Deploy to ZopDay Button

https://zop.dev/deploytozopday-inkhard.svg

Notes

• height="32" is set on the image deliberately: the Heroku button is 156×32 and the Koyeb
  button is 150×32, while the ZopDay SVG is 169×36 unscaled. Without it the three buttons would
  not line up.
• The badge is served from the production host zop.dev, so it will not break later.
• Unlike the two button-deploys above, this one does not fork the repository, so there is no CI
  wired up from it.
• Helm Charts are supported only when deploying to the user's own cloud Kubernetes cluster, not
  ZopCloud.