fix idiomatic judgment rendering in mdbook

[nikomatsakis]

What does this PR do?

Fixes the "idiomatic judgment fn" rendering the mdbook by creating a plugin that lets you reference a skill and which strips the YAML frontmatter. I'd like to (from the Symposium org, most likely) make this into a reusable crate, but for now let's just embed it here in formality.

How does it work, what questions do you have?

See above.

AI disclosure

• The AI tool authored and opened this PR autonomously

[rustbot]

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