docs: recommend Node.js built-in proxy support for corporate proxies (#33006)

Node.js 22.21.0+ natively supports HTTP_PROXY, HTTPS_PROXY, and
NO_PROXY environment variables via NODE_USE_ENV_PROXY, eliminating the
need for undici and global-agent workarounds. This also works with
node-fetch and cross-fetch since they delegate to node:http/node:https
without overriding the HTTP agent.

Add a new corporate proxy tutorial under docs/ with the recommended
approach and update the legacy guide in contrib/ to point to it.
Update proxy references in the deployment, keeping-backstage-updated,
and TechDocs CLI docs to mention NODE_USE_ENV_PROXY.

Signed-off-by: Jon Koops <jonkoops@gmail.com>

docs/deployment/index.md

@@ -32,6 +32,4 @@ This method is covered in [Building a Docker image](docker.md) and
 
 There are many ways to deploy Backstage! You can find more examples in the community contributed guides found [here](https://github.com/backstage/backstage/blob/master/contrib/docs/tutorials/).
 
-If you need to run Backstage behind a corporate proxy, this
-[contributed guide](https://github.com/backstage/backstage/blob/master/contrib/docs/tutorials/help-im-behind-a-corporate-proxy.md)
-may help.
+If you need to run Backstage behind a corporate proxy, see the [corporate proxy guide](../tutorials/corporate-proxy.md).