docs(release-docs): reconcile next docs changes into newly cut version

[critesjosh]

What

Rewrites Step 14 of the release-docs skill (.claude/skills/release-docs/SKILL.md) to reliably catch documentation changes that landed on next but won't be present in a version cut from the release tag.

Why

A new docs version (e.g. 5.1.0) is cut from the release tag, which is older than next. Any docs work merged into next after the tag is therefore missing from the freshly cut snapshot. The previous Step 14 gestured at this but had two gaps:

1. It only inspected source docs, never the existing versioned snapshots. A fix merged directly into the previous version's snapshot on next (e.g. developer_versioned_docs/version-v5.0.0-rc.1/...) was never reviewed.
2. It diffed the working tree against the new snapshot. Since the working tree is checked out at the release tag, that diff compared tag-source against a snapshot just cut from the same source — near-identical, and blind to what actually diverged on next.

Changes

• Reframe Step 14 ("Reconcile next Docs Changes Into the New Version") to explain the tag-vs-next divergence up front.
• Call out the two classes of missed change explicitly: source/current docs and fixes applied directly to the previous versioned snapshot.
• Diff against origin/next (not the working tree): git log v<new>..origin/next -- docs/, plus per-area diffs for source docs and the previous snapshot directory.
• Add a Key Points bullet flagging this as the most commonly missed step.