Update Cmdstan doc to remove stanc2 references

[WardBrian]

Submission Checklist

• Builds locally
• Declare copyright holder and open-source license: see below

Summary

Resolves #389
Resolves #372

This removes references to stanc2 from the cmdstan guide, as it is no longer shipped. It also adds two new flags to the list of stanc options, --info and --warn-pedantic.

Copyright and Licensing

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Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bob-carpenter]

Thanks. This looks good, so I'm approving. So feel free to merge.

Before merging, I thought I'd ask if you think it makes sense to remove all reference to the "3" in "stanc3"? That's more of a developer issue and it seems a bit confusing to alternately talk about stanc and stanc3 in the doc. (I did check there were no more refs to stanc2.)

[WardBrian]

We do have this language on the installation page:

As of release 2.22, the CmdStan Stan to C++ compiler is written in
OCaml. This compiler is called "stanc3" and has has its own
repository https://github.com/stan-dev/stanc3, from which pre-built
binaries for Linux, Mac, and Windows can be downloaded.

which I feel like is sufficiently explanatory of the 3 in the name