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

[WardBrian]

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

[avehtari]

Pinging @bob-carpenter who last week asked to point out doc issues he could work on in the near future