Onprem documentation (#624)

* 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

* fix(sidebar): register Self-hosted section and topic so correct nav appears on on-prem docs

- Add dedicated 'self-hosted' sidebar entry (Getting started / Deploy Scalekit / Maintain and upgrade)
- Register '/self-hosted/**/*' in topics
- Map to developer-resources secondary nav

This ensures /self-hosted/* pages no longer fall back to the Resources/Code samples sidebar.

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Co-authored-by: Saif <saif.shaik@scalekit.com>

Author: anujsharmask