CmdStan guide to stanc out of date

[WardBrian]

Summary:

The part of the CmdStan guide on stanc still mentions stanc2, which was removed in 2.27

docs/src/cmdstan-guide/stanc.Rmd

Line 14 in 7c2dd82

is still available as program `bin/stanc2`. This compiler is no

docs/src/cmdstan-guide/stanc.Rmd

Line 146 in 7c2dd82

## Command-line options for stanc2

Additionally, they do not document all the command line flags of stanc3 (such as --warn-pedantic)

Current Version:

v2.27.0