CI/Docs: streamline doc jobs

What

1. Review binary dependencies and remove non-needed ones: oc.build_kernels and oc.build_amd64-exp.
2. Merge documentation.manuals into documentation.build_all as the script step duration of documentation.manuals is < 10s, negligeable during the time to start the pod.

Why

• Reduce costs.
• Code quality: clean and simplify code.
• There should also be potential wall-time decrease (esp. thanks to the merge of documentation.manuals).

History

Reverts partially !14937 (merged).

• Note that since !20200 (merged), the bottleneck is not anymore documentation.docgen or documentation.odoc so documentation.manuals will not start before the end of these jobs and so we don't lose anything by merging it in documentation.build_all. On the contrary, we'll be saving some time since there is one less pod to start.