Add SHOWROOM_TABS env var for runtime tab override

[prakhar1985]

Summary

• Adds content.tabs value (default: empty string) to showroom-single-pod chart
• Passes SHOWROOM_TABS env var to the antora-builder init container
• When set to a JSON array of tab objects, the antora entrypoint overrides ui-config.yml tabs before build

Follows the same pattern as ANTORA_ENABLE_DEV_MODE in #48.

Related PRs

• showroom-images: rhpds/showroom-images#37 (entrypoint configure_tabs() function)
• showroom collection: Add tabs override variable to ocp4_workload_showroom agnosticd/showroom#24 (Ansible variable plumbing)

Example usage

helm template showroom charts/showroom-single-pod \
  --set-json 'content.tabs=[{"name":"OCP Console","url":"https://console-openshift-console.apps.example.com"},{"name":"Terminal","type":"terminal"}]'

When content.tabs is empty (default), the content repo's ui-config.yml tabs are used unchanged.