Add Offline Platform install guide

[ArnobKumarSaha]

Adds a new Offline Platform install guide for air-gapped KubeDB Platform deployments.

• Offline mode is only supported with the Self Hosted Production deployment type
• Documents the Offline Installer? toggle and Cluster ID field (Cluster ID is the only data AppsCode receives, used solely for license issuance)
• Documents the extra acaas chart install step (billing frontend/backend), deployed only in offline mode
• Documents that importing additional DBaaS clusters requires the admin to obtain the per-cluster license manually from the AppsCode portal and supply it during import
• Added to the sidebar under Self Hosted Installer (weight 7)