docs(tutorials): add a tutorial for external resource references

[ehennestad]

Motivation

Add a tutorial for the HERD API, mirroring the PyNWB external resources tutorial so a reader can move between the two APIs.

Preview tutorial on ReadTheDocs:
https://matnwb--885.org.readthedocs.build/en/885/pages/tutorials/externalResources.html

What changed

• tutorials/externalResources.mlx records and reads back external resource references on a file it builds itself: a subject's species, and a column of an electrodes table.
• The tutorial has a page under General Tutorials.

Storing a HERD outside a file is left out, because to_zip and from_zip are not implemented in MatNWB.

How to test

nwbtest('Name', 'tests.system.tutorial.TutorialTest')

Todo

• Verify and approve PRs below in stack
• Verify tutorial is clear to follow and covers relevant parts of API

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

🤖 Generated with Claude Code

[ehennestad]

Pass the table as the object and the column name as Attribute. The reference is attached to the column itself, which is the object that holds the values, rather than to the table. A ragged column works the same way: Attribute names the column holding the values, and the index that groups them into rows is a separate property.

The last half of this paragraph is unclear - Either improve or drop it completely (The main point here is that the column itself is added as an attribute, not an index column)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.36%. Comparing base (0995f9e) to head (0b4c7ae).

Additional details and impacted files

@@                Coverage Diff                 @@
##           add-herd-user-api     #885   +/-   ##
==================================================
  Coverage              95.36%   95.36%           
==================================================
  Files                    237      237           
  Lines                   8660     8660           
==================================================
  Hits                    8259     8259           
  Misses                   401      401           

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