[DOCS] Add ES|QL doc structure

[marciw]

Adds an esql.asciidoc file that sets up a basic structure for the ES|QL documentation. Based on @szabosteve's Java client docs PR (#789).

This includes adding a folder structure for helpers instead of keeping the docs all on one page (but we can undo that).

@ezimuel This needs expanding and tailoring for the PHP client (it's really just placeholder text from the Java client docs). All other feedback is welcome, especially because I'm still so new to Elastic and to the client docs.

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