Create the Functional techniques overview, lambdas, local functions, and iterators articles

[Everday C# PR 22 - Functional Techniques](https://github.com/dotnet/docs/blob/EverydayCSharp/.github/projects/everyday-csharp/plan/phase-f-patterns-functional.md#pr-22--functional-techniques)

PR 22 is the committed Functional techniques slice for September—it creates the section's structural core (overview, lambdas, local functions, iterators), unlike PR 23 which is just a breadth tutorial follow-on.

Revise `functional/index.md` around C# as a multi-paradigm language. Add `functional/lambdas.md`. Pull `functional/local-functions.md` from the Programming Guide's local-functions article, and `functional/iterators.md` from existing iterator sources (harvest, don't preserve, the old locations). Add snippets, `toc.yml`, and redirects. Relocate or defer non-Fundamentals iterator detail—state-machine internals, `IAsyncEnumerable` mechanics, custom enumerator authoring, `yield` exception rules—to an appropriate non-Fundamentals home.

Done when the overview, lambdas, local functions, and iterators are live and coherent without duplicating Pattern matching, Programming Guide sources are harvested rather than left as destinations, the iterator article stays focused on simple `yield return` usage, and any redistributed material lands somewhere with redirects and inbound links intact. See the [Phase F plan](https://github.com/dotnet/docs/blob/main/.github/projects/everyday-csharp/plan/phase-f-patterns-functional.md) for scope.

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[Associated WorkItem - 632371](https://dev.azure.com/msft-skilling/Content/_workitems/edit/632371)