function `format_stan_file` missing docs, example on how to deal with #includes

[mitzimorris]

For Stan programs with include directives, format_stan_file requires the following:

STAN_INCLUDE_PATHS = [Path("stan").resolve()]
format_stan_file(m1_stan_file,  stanc_options={"include-paths" : STAN_INCLUDE_PATHS})

Document this?

Currently, because of issue stan-dev/stanc3#1621, if a stan program file has multiple includes, format_stan_file also requires option: canonicalize=includes - which is technically a stanc option, and therefore one would expect it to be part of the stanc_options dict - this doesn't seem to work.