This rips out the `PIPELINE` section from osbuild(1) and instead adds a
new osbuild-manifest(5) man-page. This new man-page contains a rather
formal definition of the manifest, with a separate section for each
part of a manifest.
The man-page is exhaustive, in that it describes all available options.
However, it does *NOT* document the available stages, runners, and
assemblers. It does document the available (and supported) sources.
This should serve as an example how to document available stages and
assemblers in the future.
Note that it is not clear whether we should document these right now.
Once we decided to support the available stages for a reasonable
time-frame, we can start on documenting them as well.
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