Add sharding_key_issue_url for gpg_key_subkeys

What does this MR do and why?

This MR updates the database documentation YAML file for the gpg_key_subkeys table to include a sharding_key_issue_url. This URL links to the issue created for defining a sharding key for this table.

The gpg_key_subkeys table currently has desired_sharding_key defined but was missing the sharding_key_issue_url attribute, which is critical for tracking the progress of sharding implementation and providing context about the sharding decision. Please note that you should also add desired_sharding_key_migration_job_name in the same MR which you queue the backfill migration for the sharding key.

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

References

Related to: #583001 (closed)

Related epic: &11670

Screenshots or screen recordings

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

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