update python perlmutter advice
- Adds a page to the Python section of the docs about Perlmutter readiness and GPU porting
- Removes the info on the Perlmutter readiness page and redirects to the new Python page
- Adds a GPU readiness pointer to the Python FAQ page
- Maybe eventually this page will move to the new Perlmutter section of the docs, but this seems like a good temporary home