By default, generate them whenever the file is missing but adding a way
to override that with `validateflow,generate-expectations=true` to force
regenerating them or setting `validateflow,generate-expectations=false`
to disallow generating them (on CI servers for example)
Also update the validateflow documentation to take that into account
and remove references to pipeline.json file which is now gone!
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-devtools/-/merge_requests/200>
