Fix missing basic usage documentation link

fix `make live-docs` reporting

```
INFO    -  The following pages exist in the docs directory, but are not
included in the "nav" configuration:
  - user-guide/basic-usage.md
```

Change-Id: Iafd4d49b063ffb676cc303135495283a125b5297

mkdocs.yml

@@ -43,6 +43,7 @@ nav:
   - User guide:
       - NGINX Configuration:
           - Introduction: "user-guide/nginx-configuration/index.md"
+          - Basic usage: "user-guide/basic-usage.md"
           - Annotations: "user-guide/nginx-configuration/annotations.md"
           - ConfigMap: "user-guide/nginx-configuration/configmap.md"
           - Custom NGINX template: "user-guide/nginx-configuration/custom-template.md"