enhancement: Decide the restart behavior for sites

[nabil1440]

Blocker: no. This can wait. This is a product decision, not a defect.

Defect

• fly restart runs docker compose restart. This command does not apply changes in the compose file or in .env.
• fly base restart runs down and then up. Thus, the two restart commands operate differently.
• fly sites restart stops all sites first. Then, it starts the sites one by one. Thus, for a period, no site operates.

Cost

• Users expect fly restart to apply configuration changes. It does not apply them.
• fly sites restart stops all sites at the same time. The downtime is longer than necessary.

Benefit of the fix

• The two restart commands operate the same way. As an alternative, the documentation tells the difference.
• The downtime for fly sites restart decreases.

Decision necessary

1. Must fly restart apply configuration changes (down and up), or must it only restart the containers?
2. Must fly sites restart restart the sites one at a time?