Tweak docs search ranking

[pokey]

Tweaks docs search so that we

• prefer to search within the same lvl0 (eg "For users" vs "For contributors"), and
• prefer docs that are not api docs, as these are a bit noisy.

Note that this PR "swizzles" a Docusaurus core class, because there doesn't seem to be any other way to create context-aware search rankings. For the actual change we made to the docusaurus core class, see 5a2a365

This PR also captures the crawler / index settings that live in the Algolia console today. See #917 for how we plan to make the files in source control the source of truth in the future. You can see the actual changes that were made in the search console to improve search ranking in b37cd16

To try out the new search rankings, pop up the search widget in the deploy preview at https://deploy-preview-915--cursorless.netlify.app/docs/

Fixes #644

Before

After

Checklist

• [ ] I have added tests