[8.14] [Request][ESS] Document advanced setting that allows users to disable ES|QL in ESS (backport #5181)

[mergify]

Contributes to #5163 and #5149

Previews: A note about the enableESQL advanced setting was added to the following locations:

• At the start of the Use ES|QL to investigate events section
  • NOTE: The outdated language that tells users to not use ES|QL in production envs, and the note about the ES|QL Timeline tab being in tech preview, will be removed once I merge [Request][ESS][8.14] ES|QL Timeline tab enabled by default and moved to GA  #5139.
• At the start of the Knowledge base for ES|QL section
• Under the description for the ES|QL rule type here

No Serverless PR was created. The advanced setting is only available in ESS right now.

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This is an automatic backport of pull request #5181 done by Mergify.

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