net/url: add examples for URL.Clone and Values.Clone

[mrbishalbaniya]

Add four examples demonstrating the new Clone methods for URL and Values types.

ExampleURL_Clone shows how to create a deep copy of a URL and demonstrates that modifications to the clone don't affect the original.

ExampleURL_Clone_nil demonstrates that cloning a nil URL returns nil.

ExampleValues_Clone shows how to create a deep copy of Values and demonstrates that modifications to the clone don't affect the original.

ExampleValues_Clone_nil demonstrates that cloning nil Values returns nil.

Related to issue #73450.

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