Use the "sphinx-copybutton" extension in documentation

[rahulporuri]

This PR does the following -

• Use the sphinx_copybutton extension when building the sphinx documentation.
• Make sphinx-copybutton a docs extra for the traits package
• Installs sphinx-copybutton when building documentation on GitHub Actions CI
• Installs sphinx-copybutton from PyPI when building a development environment using the etstool utility.

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

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in traits-stubs

[mdickinson]

@rahulporuri Thanks for the PR.

Sphinx maintenance hasn't been completely painless in the past, and I'm a little bit nervous about adding another third-party dependency - it would be much better if there were something in one of the built-in Sphinx extensions.

That said, this should be an easy-to-maintain addition, and it's definitely valuable. Let's merge and see how this goes; if maintaining it turns out to be problematic, we may need to reconsider.

If we switch to a different theme at some point in the future, it may be that the new theme supports this out of the box.

[mdickinson]

Let's merge and see how this goes

Actually, before I do that I'm going to merge the corresponding PR on Traits Futures, since T-F has an automated doc build that'll let us see results quickly.

[mdickinson]

Results look good on T-F, both on docs.enthought.com and Read the Docs. Merging here.

[mdickinson]

LGTM