Fix search with xs to md breakpoints

On your browser in responsive mode with a width of 991px or under OR with your mobile phone:

1. Go to https://deploy-preview-36307--twbs-bootstrap.netlify.app/docs/5.1/getting-started/introduction/
2. Click on "..."
3. Click on search input → search popup appears
4. Search input in the popup now gains automatically the focus. Search is now possible.

Fixes #36306 (closed)