Add sharding key issue URLs for gitlab-main tables

What does this MR do and why?

This MR updates the database documentation YAML files for numerous tables to include a sharding_key_issue_url. This URL links to the issue created for defining a sharding key for that specific table.

This is part of the ongoing database sharding effort and helps provide context and track the progress for making each table sharding-ready.

Tables skipped due to unknown owning team/manager: dingtalk_tracker_data, instance_integrations, verification_codes

References

Closes #553661 (closed)

Screenshots or screen recordings

This is a documentation-only change and does not affect the UI. No screenshots are required.

Before	After

How to set up and validate locally

1. Check out this branch.
2. Run the database documentation validation script to ensure all YAML files are correctly formatted:

   bundle exec scripts/database/check-valid-database-docs

3. The script should pass without any errors for the modified files.

MR acceptance checklist

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