MAINT move the examples from doc/pyplots to examples and make them reproducible #7390

We have a bunch of examples that predates the sphinx scripts to generate the gallery located in doc/pyplots. For consistency, these should be moved to the example folder and we should make sure that these are reproducible.

This ticket implies:

• moving the python files from doc/pyplots to examples and update the necessary links
• make the examples "reproducible" by fixing the random seed used by the generator.
• use the numpy.random module and not the stdlib random module.