Enable sphinx-copybutton extension by default

[tsibley]

The sphinx-copybutton extension could be enabled by default for any doc project using this theme, leading to a more consistent click-to-copy of code blocks across all our Sphinx projects.

I believe the theme's setup(app) just needs to do:

app.setup_extension("sphinx_copybutton")

with sphinx-copybutton added to the theme's setup.py.

See also https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx.setup_extension.

Context:

• https://github.com/nextstrain/ncov/pull/904
• https://github.com/nextstrain/docs.nextstrain.org/commit/cc2e2a85c66e36591d83b34b7195b1eff18b3ce7