Add manual documentation of periodic Gaussians and PySCF support

[prckent]

Describe how to use #706 including a worked example.

[prckent]

If this is tied in with some updates/bug fixes to the PySCF converter and periodic Gaussian implementation, I suggest we plan on a v3.5.1 and release now. Getting the QE6 converters to users is important. A v3.5.1 will definitely be needed due to bugs in the density weights found by #925 and #934 and reported by @kayahans