PyData Sphinx theme: DocSearch not showing on narrow screens

Split off from issue #33.

The Pydata Sphinx theme includes the searchbox twice (you can specify the template with `html_theme_options.navbar_persistent`). Once for wider screens and once for narrower screens.

The DocSearch UI can only be added once per page, so it's added to the element for wider screens only. This means the search is not available on narrower screens.

Start by reading issue #33 and tracing how the PyData Sphinx theme handles the two searchbox placements selected by html_theme_options.navbar_persistent. Verify the DocSearch integration on both wide and narrow layouts; done means the search is available on narrow screens without adding the DocSearch UI more than once per page.

Written by the indexing model from the issue text.