fix: add self-host docs service

[furtherref]

Summary

• add a standalone Docker image for the docs app
• include a docs service in the self-host Docker Compose stack and wire the web rewrite to it
• publish a multica-docs GHCR image in the release workflow

Test plan

• git diff --check
• docker compose -f docker-compose.selfhost.yml -f docker-compose.selfhost.build.yml config --quiet
• STANDALONE=true pnpm --filter @multica/docs build
• pre-push hook: pnpm lint, pnpm typecheck, pnpm test, pnpm build

Notes

• Local Docker image build is still blocked by Docker Hub access from OrbStack: resolving node:22-alpine times out while loading metadata from registry-1.docker.io.