Create the Pattern matching overview, basic pattern articles, and final Fundamentals TOC placement

[Everyday C#: PR 19 — Pattern matching: overview + declaration/constant/var + type patterns](https://github.com/dotnet/docs/blob/EverydayCSharp/.github/projects/everyday-csharp/plan/phase-f-patterns-functional.md#pr-19--pattern-matching-overview--declarationconstantvar--type-patterns)

PR 19 is the committed Phase F starting point: it makes the Patterns node live in its final Fundamentals position, now that the prerequisite Expressions and Statements work is merged. Land the overview and first basic-pattern articles together with the TOC move so the section doesn't go live as an empty placeholder.

Revise `pattern-matching.md` as the high-level introduction to pattern matching and switch expressions. Add `declaration-constant-var-patterns.md` and `type-patterns.md`, plus snippets, `toc.yml` entries, and redirects from the former `functional/` pattern paths. Reposition the Patterns TOC section into its final location above Expressions and Statements, and add the deferred Expressions-overview cross-link now that Pattern matching is live.

Done when Patterns appears as a non-empty §11 TOC node, the overview/basic-pattern articles are accessible and example-heavy, redirects preserve old paths, and the cross-link points only to the live target. See the [Phase F plan](https://github.com/dotnet/docs/blob/main/.github/projects/everyday-csharp/plan/phase-f-patterns-functional.md) for the authoritative scope.

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[Associated WorkItem - 632369](https://dev.azure.com/msft-skilling/Content/_workitems/edit/632369)