Add more in-depth description of Pathfinder to the reference-manual #751

The current page is at https://mc-stan.org/docs/reference-manual/pathfinder.html,
source: https://github.com/stan-dev/docs/blob/master/src/reference-manual/pathfinder.qmd

There is not currently a good description of the algorithmic tuning parameters or stages of the algorithm, especially compared to e.g. HMC at https://mc-stan.org/docs/reference-manual/mcmc.html

@bob-carpenter said he could do a pass on this