docs(self-hosted): rebase on-prem documentation properly

- Full self-hosted section content and assets from latest on-prem work
- Remove staging distribution portal URL (repack.scalekit.cloud)
- Replace hardcoded registry with dynamic REGISTRY_SERVER / env config
- Fix smtp.password key and gateway.provider in quickstart
- Reorder Self-hosted for precedence, fix duplicate orders (config=4, troubleshooting=8)
- Shorten quickstart sidebar label
- Polish deployment agent, token expiry messaging, troubleshooting diagnostics and Slack link
- Add opening paragraph in troubleshooting

This replaces the previous merge-polluted history (volta, heavy hooks, tooling-updates merge) with a clean base on main + the proper on-prem docs only.

Backup of previous state: onprem-documentation-backup

Author: saif-at-scalekit