Create the Functional techniques tutorial follow-on

[Everyday C# PR 23 — Tutorial: Functional techniques in C#](https://github.com/dotnet/docs/blob/EverydayCSharp/.github/projects/everyday-csharp/plan/phase-f-patterns-functional.md#pr-23--tutorial-functional-techniques-in-c)

Add `fundamentals/tutorials/functional-techniques.md` with supporting snippets and a `toc.yml` entry. Build one cohesive, breadth-first tutorial demonstrating lambda expressions, local functions, pattern-matching expressions, iterators, and LINQ together—linking to the depth articles for detailed rules rather than reteaching them. Keep the scenario small and runnable; don't drag in advanced iterator mechanics or LINQ provider internals.

Done when the tutorial is scenario-based and breadth-focused (not another reference article), every concept used links to a live canonical target, samples are compact and runnable, and completing it cleanly closes out Phase F.

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[Associated WorkItem - 632372](https://dev.azure.com/msft-skilling/Content/_workitems/edit/632372)