Add Client Examples to Enterprise Search Documentation

We have code examples for each client in the Elasticsearch documentation. For example [`bulk`](https://www.elastic.co/guide/en/elasticsearch/reference/master/docs-bulk.html) and for [`cloud`](https://www.elastic.co/guide/en/cloud/current/ec_connect_a_client.html).

We should add client usage examples to the Enterprise Search documentation too., e.g. [indexing documents](https://www.elastic.co/guide/en/workplace-search/current/workplace-search-custom-sources-api.html#index-and-update).