cubicweb-processing #3096537 Document bare-bones usage [done]

Document "programmatic" Run creation, chaining and execution, by using the basic API of the cube.

prioritynormal
typeenhancement
done in<not specified>
load left0.000
closed by#966b87f6a502 [doc] Add basic usage tutorial and example code (closes #3096537).
patch[doc] Add basic usage tutorial and example code (closes #3096537). [applied]