GA: enable distributed runners by default (keep labs flag as escape hatch)

After distributed runners have baked on Garden:

1. Enable distributed runners by default for all clusters (no longer requires the --labs distributedrunners opt-in)
2. Ship in a release
3. Update docs with runner setup guide (install, join, manage lifecycle)
4. Keep the --labs flag / gating in place as an escape hatch — an operator who sees badness can disable and fall back to the old path

Explicitly not in this issue: removing the labs flag or gating code. That waits at least one release (see follow-up).