feat: docs website (#80)

Deploy the documentation to GitHub Pages using `mdbook` and a little
more content for it. Can be visited at https://apricote.github.io/hcloud-upload-image.

docs/reference/cli/hcloud-upload-image_cleanup.md

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+## hcloud-upload-image cleanup
+
+Remove any temporary resources that were left over
+
+### Synopsis
+
+If the upload fails at any point, there might still exist a server or
+ssh key in your Hetzner Cloud project. This command cleans up any resources
+that match the label "apricote.de/created-by=hcloud-upload-image".
+
+If you want to see a preview of what would be removed, you can use the official hcloud CLI and run:
+
+    $ hcloud server list -l apricote.de/created-by=hcloud-upload-image
+    $ hcloud ssh-key list -l apricote.de/created-by=hcloud-upload-image
+
+This command does not handle any parallel executions of hcloud-upload-image
+and will remove in-use resources if called at the same time.
+
+```
+hcloud-upload-image cleanup [flags]
+```
+
+### Options
+
+```
+  -h, --help   help for cleanup
+```
+
+### Options inherited from parent commands
+
+```
+  -v, --verbose count   verbose debug output, can be specified up to 2 times
+```
+
+### SEE ALSO
+
+* [hcloud-upload-image](hcloud-upload-image.md)	 - Manage custom OS images on Hetzner Cloud.
+