Added feature: Added IsNil() Method for uuid. Also updated the Nil variable and Namespace constants documentation to reference RFC 9562 (the current standard obsoleting RFC 4122).

[hamid-husain]

This PR adds a convenience method IsNil() to the UUID type.

While it is currently possible to check for a nil UUID by comparing against the package-level Nil variable, providing an IsNil() method is more idiomatic in Go and improves code readability when performing validation checks.

Example:

u, _ := uuid.Parse(input)
if u.IsNil() {
    // handle nil uuid
}

Also updated the Nil variable and Namespace constants documentation to reference RFC 9562 (the current standard obsoleting RFC 4122).