Document deployment type and its possible values in billing and standard context documentation

Problem

Currently, we do not have a clear way to identify deployment type. realm is very limited and is showing false value in many cases.

We need new field deployment_type that should help us distinguish this.

Once, it is documented we need to update documentation for additional field and recommend that to use it for data analysis instead of realm

Solution

• Update standard context documentation
• Update this field in billable schema payload and check https://gitlab.com/gitlab-org/architecture/usage-billing/design-doc/-/issues/34 if ADR is present. Update this new field in that ADR.