Add release docs linkcheck gate

[rogerchappel]

Summary

• add an all-green release-docs fixture and runnable gate demo
• document the passing CI-style workflow
• add a grounded promo brief for showing the happy path alongside failure fixtures

Verification

• npm ci
• bash demo/run-release-docs-gate.sh

Notes

• npm ci reported 5 audit findings in the existing dependency tree: 3 moderate, 1 high, 1 critical.