Search automatically on ?q= URL query parameter

[jhawthorn]

Fixes #1319 (I wanted this for exactly that reason 😅)

Automatically start searching when ?q= parameter is given in the URL, selecting and visiting the first match if available.

If not available, the query will be filled into the search box but we won't navigate away.

This should enable configuring a generated rdoc page as a search engine in one's web browser, or as a bang command in a search engine. For reference Rails' sdoc (which I think rdoc's search is originally based on) does this: https://api.rubyonrails.org/?q=changed%3F

[matzbot]

🚀 Preview deployment available at: https://bfd86835.rdoc-6cd.pages.dev (commit: bd3ba77)

[st0012]

Accepting ?q= will be a big improvement ❤️
But is it possible to keep the search results open even after jumping like SDoc does?
If that's too complicated (I tried it a bit but failed) due to how darkfish works, I'd prefer not jumping immediately (or only jump when there's exactly one result), so we can show the results dropdown.