Professionalize TensorLib README

[Copilot]

The repo needed a professional README that clearly communicates scope, usage, and project layout. This update focuses on making the entry documentation concise, structured, and immediately actionable.

• Overview & scope: Expanded the top-level description to position TensorLib and its intended use cases.
• Capabilities & layout: Added feature highlights and a repo layout section to surface core modules and utilities.
• Getting started: Documented build, tests, benchmarks, and app entry points to reduce onboarding friction.
• Usage example: Included a minimal snippet to show the public API surface.

#include <tensorlib/ops.h>
#include <tensorlib/tensor.h>

int main() {
    auto a = Tensor::createOnes({2, 3});
    auto b = Tensor::createOnes({3, 2});
    auto c = TensorOps::matmul(a, b);
    return 0;
}