Document how to implement a new workflow in debusine

We expect some Debian developers to have ideas of interesting workflows to try out. Since we can't yet provide them a feature to create custom workflows directly in the system, we are going to document how to add a new workflow in the codebase. We have frequent releases so any newly-contributed workflow shall be shortly available.

We need to:

create a new "howto" page for this (below "Contribute to debusine" ?)
create a new "explanation" page about "Workflow orchestration" (probably needs to integrate the explanation from reference/workflows/advanced-sub-workflows at the same time)
create a new "reference" page with the relevant API documentation that one would use in a Workflow orchestrator (or extend the one about WorkRequest in the Debusine internals section)

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