[Request][ESS][8.14] Update docs to convey release state of AI Assistant's ES|QL-query-generating functionality

[nastasha-solomon]

Addresses #5098 and contributes to #5149.

Preview: Knowledge base - Made two minor changes:

• Reverted the tech preview note at the start of the "Knowledge base" section to the default language, so it's clear that the Knowledge base tab is still in beta.
• Added a note to the Knowledge base for ES|QL section that lets users know they should double-check ES|QL queries generated by the Assistant.

Twin Serverless PR: https://github.com/elastic/staging-serverless-security-docs/pull/339

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