Update cross_referencing.md

[internetisaiah]

Summary

Since Liquid's {% tab %} tag does not support reference-style links, I have removed those options from the contributing docs and added a relevant note.

Related PRs, issues, or features (optional)

• N/A

Feature release date (optional)

• N/A

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• I confirm that my PR meets the following:
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Thanks for contributing! We look forward to reading your work.

[rachel-feinberg]

Lgtm