[DOCS] Adds the structure of the ES|QL docs

[szabosteve]

Overview

This PR adds an esql.asciidoc file to the docs that contains the basic structure of the ES|QL documentation. As the helpers are documented under the Usage section in the Java book, the ES|QL documentation is proposed to be under the Usage section.

@swallez Every feedback is welcome (placement, structure, content, titles, etc). Please let me know what you think!

Preview

ES|QL in the Java client

[swallez]

@l-trotta can you create sample code for the various sections? Some of it can be imported from the blog post you wrote.

[l-trotta]

code examples added! just one thing, @swallez correct me if I'm wrong, but since we don't have an adapter for the columnar format I think we should move its explanation so that it's not grouped with the adapters.

[swallez]

@l-trotta indeed. This PR served as a blueprint for all clients documentation. Let's remove the columnar format for now and re-add it once we have the Arrow integration.

[l-trotta]

@szabosteve ready!