NDArray documentation + site reorganisation into subfolders

[Copilot]

Adds NDArray cheatsheet and runnable mdoc examples, and reorganises existing vector/matrix docs into their own subfolder.

New: site/docs/ndarray/

• cheatsheet.md — NDArray vs NumPy vs MATLAB comparison tables covering construction, properties (ndim, numel, shape, strides, isColMajor…), indexing, slicing/views, reshape/squeeze/flatten, element-wise and unary ops, in-place ops, comparisons, and broadcasting. Calls out vecxt's explicit-broadcast-only design and column-major default.

• examples.md — mdoc-compiled examples demonstrating the full API:

// Zero-copy views share backing data
val m = NDArray(Array.tabulate(12)(_.toDouble), Array(3, 4))
val s = m.slice(0, 1, 3)         // view of rows 1–2
s.data eq m.data                 // true

// Transpose is also zero-copy
val t = m.T                      // strides permuted, no copy

// Explicit broadcasting before binary ops
val bias  = NDArray(Array(10.0, 20.0, 30.0), Array(1, 3))
val (d2, b2) = broadcastPair(NDArray.fill(Array(4, 3), 0.0), bias)
val result = d2 + b2

Reorganised: site/docs/vectors-and-matrices/

Moved cheatsheet.md, examples.md, matrix.md, and higherkinded.md from the docs root into this subfolder. General docs (Setup.md, bounds.md, gotcha.md, index.md) remain at the top level.