docs: update quick-start for issue 3130 (#3134)

This PR clarifies the quick start documentation in regards to registry
username and password variables and removes reference to NGC since end
users most likely will pull from their own registry

## Related issues
#3130

## Type of Change
<!-- Check one that best describes this PR -->
- [ ] **Add** - New feature or capability
- [ ] **Change** - Changes in existing functionality
- [ X] **Fix** - Bug fixes
- [ ] **Remove** - Removed features or deprecated functionality
- [ ] **Internal** - Internal changes (refactoring, tests, docs, etc.)

## Breaking Changes
<!-- If checked, describe the breaking changes and migration steps -->
<!-- Breaking changes are not generally permitted, please discuss on a
GitHub discussion or with the development team if you believe you need
to break a backward compatibility guarantee -->
- [ ] **This PR contains breaking changes**

## Testing
<!-- How was this tested? Check all that apply -->
- [ ] Unit tests added/updated
- [ ] Integration tests added/updated
- [ ] Manual testing performed
- [ X] No testing required (docs, internal refactor, etc.)

## Additional Notes
<!-- Any additional context, deployment notes, or reviewer guidance -->

Signed-off-by: Benjamin Schmaus <bschmaus@nvidia.com>

Author: schmaustech

docs/getting-started/quick-start.md

@@ -77,17 +77,16 @@ Everything in this step must be done **before** running `setup.sh`. Skipping any
 ### 3a. Set Required Environment Variables
 
 ```bash
-export KUBECONFIG=/path/to/kubeconfig          # your cluster kubeconfig
-export REGISTRY_PULL_SECRET=<pull-secret-or-api-key>  # your registry pull credential
+export KUBECONFIG=/path/to/kubeconfig                 # cluster kubeconfig
+export REGISTRY_PULL_USERNAME=<registry-user>         # registry username
+export REGISTRY_PULL_SECRET=<registry-user-pass>      # registry username password
 export NICO_IMAGE_REGISTRY=my-registry.example.com/nico  # base registry for all NICo images
-export NICO_CORE_IMAGE_TAG=<nico-core-image-tag>  # e.g. v2025.12.30-rc1
+export NICO_CORE_IMAGE_TAG=<nico-core-image-tag>      # e.g. v2025.12.30-rc1
 export NICO_REST_IMAGE_TAG=<nico-rest-image-tag>      # e.g. v1.0.4
 ```
 
 `NICO_IMAGE_REGISTRY` is used for both NICo Core (`<registry>/nvmetal-carbide`) and NICo REST (`<registry>/nico-rest-*`). Push all images to this registry before running setup.
 
-Obtain an NGC API key at [ngc.nvidia.com](https://ngc.nvidia.com) → **API Keys** → **Generate Personal Key**.
-
 | Variable | Required | Description |
 |----------|----------|-------------|
 | `REGISTRY_PULL_SECRET` | **Yes** | Pull secret and API key for your image registry. Used to create the image pull secret for both NICo Core and NICo REST. |