Add `in:` operator in search

[hyunbinseo]

Follow-up for the following PR:

• Add 'docs:' search filter #666

Related to these issues, but this is more of a feature request.

• Docs search overly prioritizes tutorials #1420
• Option to exclude Tutorials from search results on svelte.dev #661

I think an in: operator would also be useful though is less low-hanging fruit (mostly because of the discoverability requirement). I don't think we should make things convoluted with saved preferences and tabs and whatnot.

I am stressed with accidentally clicking on tutorial and example links.

e.g. searching for param results in tutorial and examples on the top.

I just want a reliable way to search docs only, as the original PR author did.

The in: operator doesn't have to be included in the GUI - it can be a hidden feature.

I use custom search engine to search for Svelte docs, so simply adding it in the URL will solve my problem:

- https://svelte.dev/search?q=%s
+ https://svelte.dev/search?q=%s&in:docs