docs: spindle: hosting: create log directory

Fixes error:

```
time=2025-07-13T15:01:04.268Z level=WARN msg="failed to setup step logger; logs will not be persisted" component=spindle error="creating log file: open /var/log/spindle/knot1.tangled.sh-3ltu5bpvgwt22-fix-todos.yaml.log: no such file or directory"
```

Signed-off-by: John Andersen <[e-mail]>

docs/spindle/hosting.md

···
     go build -o cmd/spindle/spindle cmd/spindle/main.go
     ```
 
-3.  **Run the Spindle binary.**
+3.  **Create the log directory.**
+
+    ```shell
+    sudo mkdir -p /var/log/spindle
+    sudo chown $USER:$USER -R /var/log/spindle
+    ```
+
+4.  **Run the Spindle binary.**
 
     ```shell
     ./cmd/spindle/spindle