* [ADD] subscriptions: add base documentation for Subscriptions
* [ADD] subscriptions/sales_flow/create_a_quotation: new content
* [ADD] subscriptions/sales_flow/renewals: new content
* [ADD] subscriptions/sales_flow/upselling: new content
* [ADD] subscriptions/sales_flow/closing: new content
* [ADD] subscriptions/reporting/subscription_reports: new content