FR: Pass focus to site's search bar field after user presses `/`

[arthur-shaw]

This is a (nerdy Vim-derived) convention for several sites. For example:

• G-Suite (e.g., Gmail, Drive, etc.)
• Quarto docs
• Docusaurus docs (despite the site advertising the Slack-friendly Ctrl + K)
• GraphQL docs (ditto Docusaurus observation)

For non-nerdy visitors to the site, it's nice to have a good way to quickly do the thing they came to do: search for an article.

For the nerdy, it's nice to have Vim-inspired keyboard shortcuts.