Add deployment tutorial for fly.io (#2252)

docs/content/documentation/deployment/flyio.md

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+title = "Fly.io"
+weight = 70
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+
+If you don't have an account with fly.io, you can sign up [here](https://fly.io/app/sign-up).
+
+Then install the `flyctl` tool following the instructions [here](https://fly.io/docs/hands-on/install-flyctl/).
+
+Create a `Dockerfile`:
+
+```Dockerfile
+FROM ghcr.io/getzola/zola:v0.17.2 AS builder
+
+WORKDIR /app
+COPY . .
+RUN ["zola", "build"]
+
+FROM joseluisq/static-web-server:2
+COPY --from=builder /app/public /public
+ENV SERVER_PORT 8080
+```
+
+You can now run `fly launch`. It will detect the `Dockerfile` and mostly auto-configure everything. Fill out the necessary information, but say "no" to (1) launching any databases and (2) deploying immediately.
+
+Take note of the hostname assigned to your app.
+
+If you already have a Zola site you must now ensure that `base_url` in `config.toml` is set correctly using the hostname from your app (or whatever domain you have pointing to the app):
+
+    base_url = "https://white-snow-9922.fly.dev"
+
+If you don't have an existing site, initialize one with `zola init -f` and remember to set the correct `base_url`.
+
+You're now ready to launch your site! Run `flyctl deploy` and have fun!
+
+Finally, to set up continuous deployment of your site from GitHub, follow [this](https://fly.io/docs/app-guides/continuous-deployment-with-github-actions/) guide, steps 4-8. Any changes to your site will now be pushed automatically.