gh-38503: Reviewer guide: Link to Eric Gourgoulhon's test notebook
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes #12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes #12345". -->
https://groups.google.com/g/sage-devel/c/vFsNl87zLxo/m/yce6yB_TBwAJ

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
    
URL: #38503
Reported by: Matthias Köppe
Reviewer(s):

Author: Release Manager

src/doc/en/developer/review.rst

@@ -76,6 +76,13 @@ The following should generally be checked while reading and testing the code:
   optional doctests related to the functionality. See :ref:`chapter-doctesting`
   for more information.
 
+For changes that affect the **user interface**, in particular, upgrades to
+IPython and Jupyter component packages, manual testing is crucial because
+our automatic tests do not cover the user interface. We recommend to use
+a `Jupyter notebook with comprehensive tests of graphics and typesetting
+<https://github.com/egourgoulhon/SageMathTest/blob/master/Notebooks/test_display.ipynb>`_,
+some of which is Sage-specific.
+
 You are now ready to change the PR's status (see
 :ref:`section-github-pr-status`):