Use the "sphinx-copybutton" extension in documentation

[rahulporuri]

This PR does the following:

• Use the sphinx_copybutton extension when building the Sphinx documentation
• Create a docs extra for the pyface package and add sphinx-copybutton as a docs dependency along with enthought-sphinx-theme and sphinx
• Install sphinx-copybutton from PyPI when building docs using the etstool.py utility.

Ref https://sphinx-copybutton.readthedocs.io/en/latest/