message: docs: Install nextstrain.sphinx.theme extension
  The new extension has sphinx_copybutton pre-installed and configured. This change enables it here.
  This also makes it easier to configure other extensions across multiple docs projects.
diff: @@ -38,7 +38,8 @@ extensions = [
  'sphinx_markdown_tables',
  'sphinxarg.ext',
  'sphinx.ext.autodoc',
  - 'sphinx_tabs.tabs'
  + 'sphinx_tabs.tabs',
  + 'nextstrain.sphinx.theme',
  ]
  # Add any paths that contain templates here, relative to this directory.
  