row/col stochastic matrix documentation

[SteveBronder]

Adds docs for row and column stochastic matrix types.

I tried not to repeat myself too much by doing something similar to what Eigen does for csr matrices and csc matrices. I mostly explained the column_stochastic_matrix type and noted the differences for row_stochastic_matrix when necessary. Is that alright?

Submission Checklist

• [x] Builds locally
• [ ] New functions marked with <<{ since VERSION }>>
  • Is there an example of this in the docs somewhere?
• [x] Declare copyright holder and open-source license: see below

Summary

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): 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-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/)