Add a reference to mdast-util-hidden

[Xunnamius]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests

Description of changes

Available here: https://github.com/Xunnamius/unified-utils/tree/main/packages

This is a utility that lets you hide mdast nodes from transforms (i.e. stash them away as the "hidden children" of a Hidden node) and then reveal them later.

Useful for plugins like remark-ignore and remark-renumber-references where it's necessary to hide nodes while executing third-party/vendor transformations.

[wooorm]

Thank you! Cool idea :)