Update System.Numerics.Tensors PACKAGE.md to reflect current library capabilities

[Copilot]

The PACKAGE.md file for System.Numerics.Tensors was outdated and no longer accurately described the library's current capabilities. The documentation incorrectly suggested the library only worked with ReadOnlySpan<float> and only provided TensorPrimitives, when in fact the library has evolved to include a comprehensive set of generic tensor types and operations.

Changes Made

Updated Library Description:

• Changed from describing operations on "tensors represented as spans" to describing both "high-level tensor types and low-level primitives for working with multi-dimensional numeric data"
• Clarified that the library offers both approaches for different use cases

Enhanced Key Features:

• Added mention of high-level tensor types: Tensor<T>, TensorSpan<T>, ReadOnlyTensorSpan<T>
• Highlighted generic support for various numeric types (not just float)
• Maintained existing feature descriptions while adding context about SIMD acceleration

Improved Example Code:

• Kept the original TensorPrimitives example to show continuity
• Added new example demonstrating high-level Tensor operations with multi-dimensional arrays
• Both examples compile and work correctly with the current API

Updated Main Types Section:

• Expanded from listing only TensorPrimitives to include all major types:
  • TensorPrimitives - Low-level span operations
  • Tensor<T> - Generic tensor class
  • TensorSpan<T> - Mutable span-like view
  • ReadOnlyTensorSpan<T> - Read-only span-like view
  • Tensor - Static class with high-level operations

The updated documentation now accurately reflects the library's evolution from a simple span-based primitives library to a comprehensive tensor computation framework while maintaining backward compatibility information.

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[Copilot]

Pull Request Overview

Updates the System.Numerics.Tensors PACKAGE.md documentation to accurately reflect the library's current capabilities, expanding from a description focused solely on span-based primitives to include the comprehensive tensor types and operations now available.

• Updated library description to highlight both high-level tensor types and low-level primitives
• Enhanced examples to demonstrate both TensorPrimitives and high-level Tensor operations
• Expanded main types section to include all major tensor types and operations

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[stephentoub]

@copilot, please fix the linting errors:

Error: src/libraries/System.Numerics.Tensors/src/PACKAGE.md:57:90 MD009/no-trailing-spaces Trailing spaces [Expected: 0; Actual: 2]

[Copilot]

@copilot, please fix the linting errors:

Error: src/libraries/System.Numerics.Tensors/src/PACKAGE.md:57:90 MD009/no-trailing-spaces Trailing spaces [Expected: 0; Actual: 2]

...

Fixed trailing spaces linting error in commit 124e127.

[stephentoub]

/ba-g markdown only