Put aiprompt_config submodule route under 'admin/config/ai' path

Problem/Motivation

There is ai → module, which is configured under admin/config/ai path, while AI prompt configuration entity does not have its own group yet.
Moving aiprompt under admin/config/ai/aiprompt path and grouping under common AI will make it easier to find. And rename menu from "AI Prompt" to "AI prompt engineering". Add help description on the page explaining what it is in comparison to AI prompt content entity which can be found on another route.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes