Enable onsite Algolia search to index external URLs

[jhaaaa]

We have SDK reference docs published to the following sites outside of xmtp.org:

• xmtp-js and xmtp-web React: https://xmtp-js.pages.dev/modules
• xmtp-ios: https://xmtp.github.io/xmtp-ios/documentation/xmtp
• xmtp-android: https://xmtp.github.io/xmtp-android/
• xmtp-flutter: https://pub.dev/documentation/xmtp/latest/xmtp/Client-class.html

We publish them to external sites instead of to xmtp.org because the autogenerated docs are formatted in widely different formats that would be difficult to reformat to fit into the xmtp.org architecture. This is especially difficult because the reformatting would have to happen automatically via script each time the docs are autogenerated based on merges to their respective repos -- and then auto-published to .org.

So instead, we autogenerate to these individual sites using the native format of each source. But how to make the content discoverable on xmtp.org? Is this something that devs would find useful?

If yes, let's use Algolia search to index external URLs so that when a dev enters a relevant search on xmtp.org, content from any of these external ref docs sites can display in the search results.

Here is the Algolia doc on how to achieve this: https://www.algolia.com/doc/guides/building-search-ui/ui-and-ux-patterns/multi-index-search/js/. To do this, we must use Algolia's InstantSearch.js, which we don't. We use algolia.config.json (https://github.com/xmtp/xmtp-dot-org/blob/main/algolia.config.json).

We have also qualified for the Algolia DocSearch program for open source projects, which includes user support. Right now, we don't have any user support.

So I gues what I'm trying to say is that to do multi-index search, we'll need to migrate to a different way of using Algolia - either via InstantSearch.js or the DocSearch program. I don't quite understand the difference and if both support multi-index search.