document minimal hardware, OS requirements

[mitzimorris]

Summary:

Somewhere in the docs we need to list the minimal requirements for running Stan.
This used to be in the CmdStan manual (I think), and on some of the interface wikis.

Description:

A search across the docs and forums failed to find a page that laid out the supported OS and compilers.
Maybe this is documented in our CI; we need to make it more widely accessible.