Add documentation about AI calls within a request

What does this MR do?

Add documentation about AI calls within a request

This documents what AI calls happen from within Sidekiq, and what to do when we want to call out to an AI provider directly from a request:

The endpoints in the code_suggestions namespace already route to the ai-assisted fleet. Other endpoints that could reach out to an AI provider should be nested under the ai_assisted namespace.

Coming from the discussion in https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24386

Related issues

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