[DOCS] Add temporary redirect for async-search (#53454)

The following API spec files contain a link to a not-yet-created
async search docs page:

* [async_search.delete.json][0]
* [async_search.get.json][1]
* [async_search.submit.json][2]

The Elaticsearch-js client uses these spec files to create their docs.
This created a broken link in the Elaticsearch-js docs, which has broken
the docs build.

This PR adds a temporary redirect for the docs page. This redirect
should be removed when the actual API docs are added.

[0]: https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/async_search.delete.json
[1]: https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/async_search.get.json
[2]: https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/async_search.submit.json

docs/reference/redirects.asciidoc

@@ -375,3 +375,8 @@ See <<slm-api-stop>>.
 === How {ccs} works
 
 See <<ccs-gateway-seed-nodes>> and <<ccs-min-roundtrips>>.
+
+[role="exclude",id="async-search"]
+=== Asynchronous search
+
+coming::[7.x]