Optional axis argument for NDArray::shape() with negative indexing - #20

This PR extends NDArray::shape() so callers can read the full shape as today or pass an axis index (including negative indices from the last dimension) to get a single dimension length, with clear errors for out-of-range axes and zero-dimensional arrays, plus tests and documentation updates.

Motivation and Context

PHP only exposes the shape as a list, so reading the last dimension or a specific axis without extra indexing gymnastics is awkward compared to tuple-style access in other ecosystems. An optional axis argument keeps the default call identical while giving a small, discoverable API that matches how axis indices already work elsewhere on the class.

What’s Changed

• shape(?int $axis = null) returning the full list<int> when omitted or null, or one int length when an axis is given.
• Negative axis indices resolved from the last dimension, with IndexException on out-of-bounds axis indices or when indexing shape on a zero-dimensional array.
• PHPDoc with a conditional return template for static analyzers.
• Unit tests covering null vs omitted behavior, positive and negative axes, bounds failures, and the zero-dimensional case.
• User-facing docs updated on the NDArray class page, API quick reference, landing comparison table, fundamentals and quick-start guides, and the NumPy migration guide where shape is discussed.

Breaking Changes
None. Existing shape() calls without arguments behave the same.