Add 'Introduced in Catch 2.6.0.' text

docs/generators.md

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 # Data Generators
 
+> Introduced in Catch 2.6.0.
+
 Data generators (also known as _data driven/parametrized test cases_)
 let you reuse the same set of assertions across different input values.
 In Catch2, this means that they respect the ordering and nesting