[Website] API docs search broken

## The Problem

The YARD docs provide a search box that worked in the past. For example, here's the search from the [1.1.0 docs](https://block.github.io/elasticgraph/api-docs/v1.1.0/):

"There are 106 results" doesn't seem accurate, but it does return matches.

On the [1.2.0 docs](https://block.github.io/elasticgraph/api-docs/v1.2.0/) and [main docs](), this is broken:

...although the "There are 11 results" might be more accurate.

On 1.2.0/main, the `Methods` search seems to work fine, and the `Class` search appears to work on the visible classes. For example, if I search on `Schema` then it changes from this:

to this:

But if I first expand some of the module/class namespaces:

...and then try searching, it matches more classes:

So I guess it's only searching from among the "visible" classes?

## How to Fix

I suspect this is a regression in `yard`, our documentation tool.

* Figure out which version it regressed in. (You can downgrade yard and run `be rake site:serve` to play around locally on the `main` docs as they are regenerated with that command).
* Report the bugs upstream to the [YARD project](https://github.com/lsegal/yard).
* Downgrade YARD here in ElasticGraph to a version which is not broken.
* Regenerate the 1.2.0 docs (by checking the code out at the `v1.2.0` tag) so they are fixed.

See #1049 for some prior art.

Note: I'd like to see all the bugs here get fixed (including this mistaken match counts) but given a choice between the counts being off and seeing results vs accurate counts but no visible results, I'd prefer the former.