Document new diagnostics

[WardBrian]

Summary:

The pages at https://github.com/stan-dev/docs/blob/master/src/cmdstan-guide/stansummary.qmd and https://github.com/stan-dev/docs/tree/master/src/reference-manual/analysis.qmd need to be updated following @mitzimorris' work on the "new" Rhat/ESS from https://arxiv.org/abs/1903.08008 in stansummary.

See
stan-dev/stan#3313
stan-dev/cmdstan#1301

[WardBrian]

@avehtari would you have the time to take a look at these?

[avehtari]

I'll check these, and hopefully have time before the next release