docs: restore absolute URL for node-renderer testing example (#3179)

## Summary

Follow-up to #3165. Now that
`react_on_rails_pro/spec/dummy/renderer/node-renderer.js` exists on
`main`, switches the testing-example link in
`docs/oss/building-features/node-renderer/js-configuration.md` from a
cross-package relative path back to an absolute GitHub URL.

The relative path was a temporary workaround so the lychee link check
would pass before the file existed at its new location. Absolute URLs
render correctly on the published documentation site (Docusaurus /
`reactonrails.com/docs`), where cross-package relative links may not
resolve.

## Test plan

- [x] Lychee pre-commit hook passes on the changed file
- [x] Prettier passes
- [x] `curl
https://github.com/shakacode/react_on_rails/blob/main/react_on_rails_pro/spec/dummy/renderer/node-renderer.js`
returns 200

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk documentation-only change that updates a single link target
with no runtime or API impact.
> 
> **Overview**
> Restores the **testing example** link in
`docs/oss/building-features/node-renderer/js-configuration.md` to an
absolute GitHub URL (instead of a cross-package relative path), so the
link resolves correctly on the published documentation site.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
10da4a9. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Updated a testing example reference link to point to an external
GitHub repository, enhancing documentation accessibility while
preserving the reference material.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: justin808

docs/oss/building-features/node-renderer/js-configuration.md

@@ -58,7 +58,7 @@ Deprecated options:
 
 ### Testing example:
 
-[react_on_rails_pro/spec/dummy/renderer/node-renderer.js](../../../../react_on_rails_pro/spec/dummy/renderer/node-renderer.js)
+[react_on_rails_pro/spec/dummy/renderer/node-renderer.js](https://github.com/shakacode/react_on_rails/blob/main/react_on_rails_pro/spec/dummy/renderer/node-renderer.js)
 
 ### Simple example: