docs: add note on npx proxy configuration

Author: kriswest

website/docs/usage.mdx

@@ -28,3 +28,9 @@ npx -- @finos/git-proxy
 ```bash
 npx -- @finos/git-proxy-cli
 ```
+
+If you use a proxy server to access npm then please note that you also need to configure your proxy for npx before running the above commands:
+
+```bash
+npx config set https-proxy <company-proxy-endpoint>
+```