troubleshooting CmdStan install page for docs

[mitzimorris]

Summary:

We need a page for troubleshooting the installation and when the install and rebuild cmdstan scripts fail, they should provide a link to this page.

Description:

I wrote the original version of these scripts and I can't get install to work for me. Pity our users. Installation is critical. Anything we can do, we should.

[WardBrian]

I think documentation around troubleshooting is difficult and brittle. Every user will be bringing a different system config and set of knowledge, which intersect in a massive space of possible problems.

We can certainly document a few common cases (e.g., if you get an error like make - command not found we can guess the user needs to install more software first), but I think improving the scripts and (mainly) improving the cmdstan installation process itself would go much further