Add the Protovalidate Extension Registry

[timostamm]

This PR updates our guidance for extension numbers for predefined rules.

Our current guidance is:

• Range 1,000 to 99,999 is for public schemas.
• Public schemas should be registered in the Protobuf Global Extension Registry.
• 100,000 to max (536,870,911) is for private schemas.

The new guidance is:

• Range 10,000 to 49,999 is for public schemas.
• Public schemas should be registered in the Protovalidate Extension Registry.
• Range 50,000 to 99,999 is for private schemas, leaning on the documentation for custom options.

Since no predefined rules have been added to the Protobuf Global Extension Registry, this is not a breaking change.

The Protovalidate Extension Registry is a simple file in docs/extensions.md, similar to the Protobuf Global Extension Registry (https://github.com/protocolbuffers/protobuf/blob/main/docs/options.md). It is specific to extensions for validate.proto, while the Protobuf Global Extension Registry is specific to extensions for descriptor.proto. Potentially, the Protovalidate Extension Registry can be converted to an extension declaration in the future.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jan 7, 2026, 10:36 AM

[jrinehart-buf]

Documentation look good to me; I added an "instructions-first" rearrangement and @timostamm has added uniqueness clarifications.