Update doc after qt backend transition

[GuillaumeFavelier]

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PyVistaQt has been slowly transitioning from PyQt5 to a Qt-backend agnostic interface but now the transition is over so the documentation (usage.rst and README.md) needs to be updated. A contribution to make clear that there is no default Qt backend is welcome.

Suggestion

• Add an installation section

Context

Discussion about allowing different Qt bindings started slightly before 0.2 (https://github.com/pyvista/pyvistaqt/issues/8) and the selected solution was to use qtpy (https://github.com/pyvista/pyvistaqt/pull/12, https://github.com/pyvista/pyvistaqt/pull/61). Then qtpy lived with the PyQt5 during this minor version until PySide testing was introduced in https://github.com/pyvista/pyvistaqt/pull/81. Finally, after another minor version of testing, 0.4 became the first version of PyVistaQt to reach this goal.

cc @adeak

[adeak]

Feel free to ping me if there's a corresponding PR and I can take a look.

[GuillaumeFavelier]

Thank you, your help is much appreciated