Draft: Add constraint and validation to web_hooks table and system hook model
What does this MR do and why?
This adds a DB constraint to the web_hooks table, an active record validation to the SystemHook model, and updates the web_hooks documentation .yml
The constraint ensures either group_id, project_id, or organization_id on the web_hooks table is not null. These ids reference GroupHooks, ProjectHooks, and SystemHooks respectively.
This is the 4th MR for the ProtocellsSharding work issue Add organization_id to the web_hooks table (#524812)
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Related to #524812