Developer manual: how to adopt the org workflow templates in CI #15688

The developer manual has no how-to for adopting the org's GitHub workflow templates (https://github.com/nextcloud/.github/tree/master/workflow-templates). digging_deeper/continuous_integration.rst links three of about 40.

It should cover: adding a template to a repo, the composer scripts the templates expect (lint, cs:check, psalm, test:unit), keeping them in sync, and how the test matrix reads the supported versions from info.xml.