Add deployment_configuration to make deployment using infra-rebuild work

Also document usage of infra-rebuild and its configuration file.

README.md

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+# nix-infra
+
+nix infrastructure configuration for CCCHH.
+
+For deployment we're using [infra-rebuild](https://git.hamburg.ccc.de/CCCHH/infra-rebuild). \
+To easily get a shell with `infra-rebuild` going, use the following command:
+
+```
+nix shell git+https://git.hamburg.ccc.de/CCCHH/infra-rebuild#infra-rebuild
+```
+
+After that you can simply run the following to deploy e.g. the git and matrix hosts:
+
+```
+infra-rebuild switch git matrix
+```
+
+By default infra-rebuild tries to use the FQDN from the nixosConfiguration of the host for deployment.
+However to override individual parts of the deployment target, a [`deployment_configuration.json`](./deployment_configuration.json) can be used.
+This is exactly what we're doing to set the default deployment user to `colmena-deploy` and have custom target hostnames for Chaosknoten hosts, since they don't have an FQDN defined in their nixosConfiguration.

deployment_configuration.json

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+{
+  "default": {
+    "targetUser": "colmena-deploy"
+  },
+  "hosts": {
+    "netbox": {
+      "targetHostname": "netbox-intern.hamburg.ccc.de"
+    },
+    "matrix": {
+      "targetHostname": "matrix-intern.hamburg.ccc.de"
+    },
+    "public-web-static": {
+      "targetHostname": "public-web-static-intern.hamburg.ccc.de"
+    },
+    "git": {
+      "targetHostname": "git.hamburg.ccc.de"
+    },
+    "forgejo-actions-runner": {
+      "targetHostname": "forgejo-actions-runner-intern.hamburg.ccc.de"
+    },
+    "eh22-wiki": {
+      "targetHostname": "eh22-wiki-intern.hamburg.ccc.de"
+    },
+    "nix-box-june": {
+      "targetHostname": "nix-box-june-intern.hamburg.ccc.de"
+    }
+  }
+}