feat: document OpenAPI client generation
As part of the related Account Activation epic some stub documentation was created for the OpenAPI generation in the project repository.
uis/devops/continuous-delivery/ci-templates#93 (closed) covers lifting this into the ci-templates repository and the guidebook. This commit is the documentation-side of that issue.
The content itself is based heavily on previously reviewed content.
Closes uis/devops/continuous-delivery/ci-templates#93 (closed)