Add SkewStudentT

[rohanbabbar04]

Closes #399

Description

• Added SkewStudentT.

• Added tests for SkewStudentT.

• Add to docs.

  • [x] Code style is correct (follows pylint and black guidelines)
  • [x] Includes new or updated tests to cover the new feature
  • [x] New features are properly documented (with an example if appropriate)
  • [x] Includes a sample plot to visually illustrate the changes (only for plot-related functions)