Allow solo indexes to be created per path

We have a documentation project with multiple "books". E.g. android, ios, web, etc.
Books may have similar titles and headings. If a user is in the android documentation, we don't want to show them ios results.

If the user is viewing a page with the /android path, there should be an option to show only results from the `/android` path, ignoring all other search results.

Similarly, If the user is viewing a page with the /ios path, there should be an option to show only results from the `/ios` path, ignoring all other search results.

And so on...

This could probably be achieved with an option called `showResultsByBasePath`, and have paths be explicitly specified (easier to implement) or implicitly determined (harder to implement I think).