Search across multiple Jekyll sites

## I want to suggest a feature (or see if anyone has a creative workaround):

Support for searching across multiple, distinct Jekyll sites

### What is your use case for such a feature?

Why? We have a WWW site that has lots of blog content, and a separate site for software developers that has API reference information, how to articles etc. I'd like searches on either site to show matches on both. Both sites are driven by Jekyll, and this plugin works well for indexing and searching when there is one site per index.

### What is your proposed API? Is this a new option? A new behavior?

How?
a) use one index for all the sites, but use a `website` property to scope the update to the index so that when a site is indexed, it only updates those records with that website. In other words it doesn't update or delete object from a different website.
b) `website` could be a facet that allows for searching one site or all sites, and also allows the building of the full URL to the search result on a different site.

Perhaps there is a way to put a hook or callout either on the query of existing object IDs or the determining of which ones to remove.

No files or tests are named. Start by tracing the plugin's existing single-site indexing flow, including how it queries existing object IDs and determines removals; done requires an agreed API and behavior for website-scoped updates, cross-site filtering, and result URLs.

Written by the indexing model from the issue text.