* Release v1.3.0 (#869)

* cleaning up some of the .rst files into a flatter hierarchy

* editing upper bound notebook so that headers don't interfere with the doc structure

* some eidts to the GPR docstring and call signature

* imrpving kernel docstrings

* improving SGPR call signature

* simplifying use of abc.abstractmethod

* reinstating a very important period

* Update gpflow/kernels.py

Co-Authored-By: jameshensman <james.hensman@gmail.com>

* improvements for kernel docstrings

* typo fix in  gpflow/models/gpr.py

Co-Authored-By: jameshensman <james.hensman@gmail.com>

* typo fix in gpflow/models/sgpr.py

Co-Authored-By: jameshensman <james.hensman@gmail.com>

* more kernel docstring improvements

* Added module level descriptions of likelihoods and kernels, fixed table in intro, and added auto-detection of modules with auto-members

* Fixed minor typos in the docs

* improving TOC by setting trtee depth to 2

* some copyright edits

* Moved documentation from .rst to module level documentation

* including missing argument in SGPR init

* Added mean function module level documentation, changed to unicode from LaTeX for readability in code