docs/source/_templates/searchbox.html

{%- if pagename != "search" and builder != "singlehtml" %}
<form class="search" action="{{ pathto('search') }}" method="get" role="search">
   <input type="text" class="search__text" id="docs-search" name="q" aria-labelledby="searchlabel" placeholder="Search Documentation" />
</form>
{%- endif %}