Platform creation in the docs

[alecandido]

There are multiple examples of platform creation in the docs, especially in lab.rst (and at least another one in experiment.rst).

Since the platform is created within a function, supposed to be invoked by Qibolab, it's often not tested. Same goes for the JSON snippets presented.

Instead, we should rather split these snippets as separate files, similarly to what is being done in #807, and test them outside Sphinx, which is easier and more practical.
We could just make a script to test them, and include it in poe test to be run after Pytest. This will make sure it's run even in the CI.