Implement proxy endpoint for ElasticPress instant results #410

The Instant Results feature that landed alongside Altis v11 is not currently fully implemented as I misunderstood what it did.

We need to implement a version of https://github.com/10up/elasticpress-proxy that works for Altis. Notably it cannot use the filesystem other than /tmp - and then it must also properly handle running on multiple servers and clearing the cache whenever the site is reindexed.

Acceptance criteria:

• Functionality of https://github.com/10up/elasticpress-proxy is replicated for Altis
• Config option for enabling instant-results feature
• Instant Results is switched off by default
• Docs added for new instant results feature including how to activate it and what it does - namely the search modal overlay. Add a screenshot as well using the twentytwentytwo theme and some demo content & tags
• Feature works on multisite
• DB queries can be cached