#9370 improvement(docs): build feature-branch docs previews only when docs change

Summary

• Add apps/docs/vercel.json with an Ignored Build Step (scripts/vercel-ignore-build.sh)
• Production and staging build every affected commit, as today, so a shared-package change that breaks the docs build still fails on staging before it reaches main
• Other preview branches build only when apps/docs changed since the branch's last successful deployment (VERCEL_GIT_PREVIOUS_SHA); "skip unaffected projects" already triggers docs previews on every @sim/db, @sim/emcn, @sim/utils or lockfile change, which is nearly every app PR
• Anything the script can't decide builds: a branch's first deployment, or a previous SHA outside Vercel's shallow clone

Type of Change

• Improvement

Testing

• Ran the script against real commits for each path: production → build, staging → build, first deployment → build, unknown previous SHA → build, docs changed → build, no docs change → skip (exit 0 with a log line)
• shellcheck and bash -n clean; biome check on vercel.json
• Behavior follows Vercel's documented ignoreCommand contract (runs in the root directory before install; exit 0 skips, 1 builds)

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)