Modify getting started to include PyPI installation.

### Description of the modifications

Right now the getting started guide does not mention that you can (and should) install the project from PyPI. We should add a reference to it.

https://github.com/ansys/ansys-tools-visualization-interface/pull/486#discussion_r3122550762

### Useful links and references

https://usd-viewer.docs.pyansys.com/version/stable/getting_started/index.html