Move ai_settings table back to gitlab_main schema

Chen Zhangrequested to merge
531356-address-cross-foreign-keys-for-ai_settings into master

What does this MR do and why?

This MR changes the database schema definition for the ai_settings table in its corresponding documentation file (db/docs/ai_settings.yml) back to gitlab_main as the decision is pending.

Additionally, a sharding_key_issue_url is added to link to the issue.

Changelog: other

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This is a backend documentation change, so no screenshots are required.

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Edited by Chen Zhang

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