[8.16] [8.16]Adds Knowledge Base page and updates AI Assistant doc (backport #6040) #6099
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Fixes #5994 and fixes #5999 by updating the AI Assistant docs and creating a new page for the AI Assistant's knowledge base features.
Previews: Knowledge base (ESS); Elastic AI Assistant (ESS)
This is an automatic backport of pull request #6040 done by Mergify.