Patch release adding vcpkg packaging support.

[matajoh]

Improvements:

• Added vcpkg port files and removed remnants of the old NuGet packaging approach

[Copilot]

Pull request overview

This PR cuts a 2.1.1 patch release focused on adding vcpkg packaging support, updating the install layout for CMake package consumption, and removing the legacy NuGet packaging templates.

Changes:

• Added a vcpkg port (ports/libnpy) plus a vcpkg overlay-port integration test project under test/vcpkg, and wired new vcpkg CI jobs.
• Updated CMake install/export paths to standard locations (include/, share/npy) and bumped version metadata to 2.1.1.
• Removed NuGet packaging template files and adjusted release notes/docs accordingly.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
vcpkg.json	Adds root vcpkg manifest metadata for consumers.
ports/libnpy/vcpkg.json	Declares the vcpkg port manifest for libnpy.
ports/libnpy/portfile.cmake	Implements vcpkg build/install logic via vcpkg_from_github + CMake.
ports/libnpy/usage	Documents how to consume the port via CMake targets.
test/vcpkg/vcpkg.json	Adds a manifest-mode vcpkg test project depending on libnpy.
test/vcpkg/port/libnpy/vcpkg.json	Adds an overlay port manifest used by CI integration tests.
test/vcpkg/port/libnpy/portfile.cmake	Overlay portfile that builds from the checked-out repo source.
test/vcpkg/CMakeLists.txt	Builds/runs a small consumer executable via find_package(npy).
test/vcpkg/main.cpp	Integration test exercising save/load through the installed vcpkg package.
CMakeLists.txt	Adjusts install interface include path and installs headers/configs to GNUInstallDirs locations.
include/npy/npy.h	Bumps library version macros to 2.1.1.
VERSION	Bumps repository version to 2.1.1.
RELEASE_NOTES	Updates release notes for the 2.1.1 vcpkg-focused patch.
CHANGELOG.md	Adds a 2.1.1 changelog entry.
README.md	Updates top-level description to remove the .NET interface mention.
.github/workflows/prgate.yaml	Adds Linux/Windows/macOS vcpkg integration jobs.
nuget/template.targets.in	Removes legacy NuGet packaging template.
nuget/template.nuspec.in	Removes legacy NuGet packaging template.
.github/skills/version-bump/SKILL.md	Adds version-bump procedure documentation.
.github/copilot-instructions.md	Adds repo-wide Copilot agent guidance and project conventions.

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