dvc.yaml: encourage manual writing/editing stages (and pipelines)

[jorgeorpinel]

The intention since DVC 2.0 is to do so (per iterative/dvc#5180 (comment)).

• State this clearly in the docs, and the why (advanced dvc.yaml features?)
  Try to use a descriptive rather than prescriptive explanation...

stage add is still clearly useful, perhaps even preferred for certain cases:

• Simple stages or beginner users e.g. dvc stage add -n test echo test

• Programmatic stage generation, esp. to "bootstrap" a pipeline structure (to manage manually later)

• How should this be reflected in docs?

[jorgeorpinel]

May be good as a best practice too (#72). In fact it was already mentioned there...

[jorgeorpinel]

How should this be reflected in docs?

I guess this is basically a dupe of #2076!