docs: add prose-update reference to fern-docs skill

[adidavid014]

What

Adds a new reference, skills/fern-docs/references/prose-updates.md, and routes to it from SKILL.md's routing table.

The reference covers how to update prose/guide pages when an API definition changes:

• Don't hand-edit the auto-generated API reference — Fern regenerates it from the spec; only touch prose/narrative pages.
• Identify which existing prose pages a spec diff makes stale (grep for endpoint/field/method names), prefer editing existing pages over creating new ones, and make minimal targeted edits.
• Cross-reference related pages, reusing SKILL.md's linking rules (links come from docs.yml + frontmatter, not file paths).
• When a change is purely additive and only the reference needs to reflect it, making no prose edits is correct.

Why

This is for the Automated Fern Writer (Scribe) update-docs-from-changes job (Linear SE-45), which on an API-spec change opens a single docs PR that updates prose and drafts a changelog. A changelog reference already existed; the prose side was the gap. This fills it.

It ships independently — agents fetch the fern-docs skill at runtime, so merging this makes the guidance live without any code change on the Scribe side.

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