[DOCS] Adds information about elastic-esql gem

[picandocodigo]

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[github-actions]

🔍 Preview links for changed docs

• docs/reference/esql.md

[pquentin]

Thanks! LGTM.

[marciw]

Looks good! I made minor suggestions for clarity and style.

[picandocodigo]

Thank you @marciw! I included all of your suggestions. I kept the capitalized name since that's the product name (it's the same in its own README). I also modified the text on the last line since I want to explicitly let users know they don't need to use the official Elasticsearch client with this tool:

- The `elastic-esql` library works independently of the {{es}} client, so you can use it alongside any client — not just Ruby.
+ The `elastic-esql` library works independently of the {{es}} client, so you can use it alongside any client — not just `elasticsearch-ruby` .

Let me know what you think, thanks!