Issue Description: Initial documentation is required for RHOSE, in particular at the Technology Preview stage

Acceptance Criteria:

• A brief but workable Introduction document, a conceptual explanation of RHOSE
• A skeletal Getting Started document, listing and, to the degree possible, detailing key user procedures for using RHOSE

Out of Scope:

• Completion of documents
• A map of all documents (provisionally planned for Introduction)
• Service Definition (to be prepared by Program Management, edited by tch writer, published as part of docs)

Additional Information:

• Introduction requires architecture information and main user aims (mostly known as of 7 April '22)
• Getting STarted requires the UI (including CLI) and API that are available for users, both to manage RHOSE and to create processors (mostly not ready as of 7 April '22); absence of this information is a potential blocker