Add docs for mc-stan.org

# Feature request

I am adding this issue to let people know that I am starting work on importing documentation for mc-stan.org. I had previously proposed this documentation on the gitr channel and got positive feedback for the idea.

## Summary

The mc-stan documentation is somewhat similar to the scikit-learn documentation, and it is split into several parts:

1. User's guide
2. Stan language reference
3. Stan functions reference
4. User guides for projects building on top of Stan
5. Case studies
6. Tutorials

I think points 1.-3. could be added to devdocs.io. I think point 3. is lowest hanging fruit, so I will start with that.

The documentation for 1-3 is located at https://github.com/stan-dev/docs

Start by reviewing the mc-stan documentation repository at https://github.com/stan-dev/docs, focusing first on the Stan functions reference identified as the lowest-hanging-fruit. Determine the documentation import scope and confirm that the selected reference, followed by the user’s guide and language reference if included, is available in DevDocs.

Written by the indexing model from the issue text.