TEMPLATE_SECRET environment variables are not documented

[lunkwill42]

Describe the bug
Through conversation with @indy-independence, we have learned that any environment variable prefixed with TEMPLATE_SECRET_, exported to the CNaaS-NMS API process, will be made available as variables in Jinja templates.

This fact is not documented in the official documentation, but it should be.

Additional context
#215 further modifies how these variables are made available to templates.