Document SwiftPM's #include <json.hpp> form and add CI coverage

Package.swift declares the json target with path: "single_include/nlohmann",
so SwiftPM consumers must write #include <json.hpp>; the <nlohmann/json.hpp>
form used throughout the rest of this documentation does not resolve. The
SwiftPM section of package_managers.md only had the summary box and did not
mention this, nor the #4650 linker problem: because the json target ships
only headers, SwiftPM/Xcode look for a json.o that is never built, so a
consumer needs at least one .cpp file of its own.

Add a usage example (docs/mkdocs/docs/integration/swift/) that depends on
the json product, includes <json.hpp>, and works around the linker problem
with its own example.cpp; verified it builds with `swift build` against a
local checkout. Also raise the deployment target from the deprecated
.watchOS(.v4) to .watchOS(.v9): Swift 6.4 warns "'v4' is deprecated: watchOS
9.0 is the oldest supported version" on every `swift package` invocation,
and `swift package dump-package` is now clean. This is the one behavior
change in this commit: it raises the minimum watchOS version the package
declares.

Add a macOS CI job that runs `swift package dump-package` (so a future
deprecation warning fails CI) and builds the new documentation example
against the checked-out repository with `swift build`, so a change to
Package.swift or the example is caught before release. No workflow
previously referenced Swift or Package.swift.

Adding an in-repository placeholder .cpp to the library itself, so the
issue does not occur for consumers at all, was proposed and declined in
#4650; the documented workaround is a consumer-side .cpp file instead.

#5716 item 3

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

Package.swift

@@ -6,7 +6,7 @@ import PackageDescription
 let package = Package(
     name: "nlohmann-json",
      platforms: [
-        .iOS(.v12), .macOS(.v10_13), .tvOS(.v12), .watchOS(.v4), .visionOS(.v1)
+        .iOS(.v12), .macOS(.v10_13), .tvOS(.v12), .watchOS(.v9), .visionOS(.v1)
     ],
     products: [
         .library(name: "json", targets: ["json"])