Adopt the shared docs-check workflow

[cpitzi]

Adopts the reusable docs-check workflow from lentago/shared-workflows#28, which resolves relative markdown links across the repo's git-tracked markdown and fails on genuinely broken ones.

The motivation is lentago/.github#57: documentation is most of what the fleet ships, and renames and removals — its most common change class — are exactly what silently break relative links. The evidence there was a broken image on a public README that survived a deliberate rename audit, because nothing verified it.

The workflow is deliberately not path-filtered. It is built to serve as a required status check, and a required check whose workflow never triggers is held "Expected" forever and deadlocks every non-matching PR, per the standing rule in fleet-ops/required-checks.json. The checker is cheap and skips fast when no markdown changed.

Two link classes are skipped by rule rather than by hand-maintained ignore lists — the two that accounted for 237 of the 240 raw failures in the fleet-wide scan: site-absolute router routes, which an Astro/Starlight router resolves at build time, and links that escape the repo root, which is GitHub's ../../issues/N navigation convention and can never point at a tracked file. This repo needs no additional ignore entries: it was scanned with the checker before this PR was opened and reports clean.

Part of a fleet-wide rollout; the matching required-checks.json mapping follows as a separate change in the .github meta-repo once the context name is observed off a live check-run.