Install nextstrain.sphinx.theme extension downstream

[victorlin]

Upon seeing nextstrain/cli#198 I noticed we never leveraged #24 in practice across our docs projects.

• readthedocs-testbed: Install nextstrain.sphinx.theme extension readthedocs-testbed#1
• nextstrain.org: docs: Install nextstrain.sphinx.theme extension nextstrain.org#574
• nextclade: docs: Install nextstrain.sphinx.theme extension nextclade#943
• cli: docs: Enable the copybutton extension we enable with our Sphinx theme cli#198
• auspice: docs: Install nextstrain.sphinx.theme extension auspice#1535
• docs.nextstrain.org: Replace sphinx_copybutton with nextstrain.sphinx.theme docs.nextstrain.org#119
• augur: docs: Install nextstrain.sphinx.theme extension augur#1005
• ncov: docs: Replace sphinx_copybutton with nextstrain.sphinx.theme ncov#983

Doing this would also automatically enable any other extensions we install here in the future.

[tsibley]

Thanks for enumerating this before I got a chance to! Saved me some trawling. :-)

[victorlin]

This checks off the "tend to smaller issues/PRs" in my task list today!