DOC: stats: Maintenance work on the distribution docs in the stats tutorial.

* Added links to implementations in the list of distributions in the tutorial.
* Removed distributions from the .rst files that are not implemented in scipy:
    continuous.rst:
        Doubly Non-central F
        Doubly Non-central t
        Noncentral beta
        Noncentral chi
        Studentized Range
        Symmetric Power
        Wishart
        Student Z
    discrete.rst
        Discete Gaussian
* Fixed spelling of "Saunders" in "Birnbaum-Saunders".
* Removed extraneous blank lines.
DOC: stats: Maintenance work on the distribution docs in the stats tutorial.

Created a separate .rst file for each distribution.
DOC: stats: Fix whitespace in the LaTeX formulas of some distributions.
DOC: stats: Remove extraneous LaTeX brackets from discrete.rst.
Merge pull request #4008 from WarrenWeckesser/doc-split-distr

DOC: stats: Split the descriptions of the distributions in the tutorial into separate files.