[API]: Add LLM instructions to our API docs and client docs

### Was the documentation helpful?

Yes

### What documentation page or section is affected

All API docs I think, like:

* https://www.elastic.co/docs/api/doc/elasticsearch/
* https://www.elastic.co/docs/api/doc/kibana/

But also all clients if possible:

* https://www.elastic.co/docs/reference/elasticsearch-clients

### Description

I found [this page](https://developer.spotify.com/documentation/web-api/tutorials/building-with-ai) from Spotify this morning and found that's it's super nice.

Could we add similar LLM instructions to our API docs like https://www.elastic.co/docs/api/doc/elasticsearch/ and within each specific client (https://www.elastic.co/docs/reference/elasticsearch-clients)?