Documentation overhaul: README, Google-style docstrings, usage guides, API reference fixes

[Copilot]

Full documentation rewrite so a new user can install, import, and use all 4 public classes without guessing. Fixes incorrect API signatures in the reference docs, adds missing guides, and standardizes docstring format.

README.md

• Restructured with: Overview, Installation, Quick Start, Core Concepts, Examples, Optional Dependencies, Development, License
• Removed speculative/conceptual sections; every code snippet is verified executable

Docstrings (Google-style)

• Converted Node, QuaternionDepthIterator, DiscreteOrientationSet, DiscreteQuaternionIterator from NumPy-style to Google-style (Args:/Returns:/Raises:/Example::)

API Reference (docs/09-api-reference.md)

• Fixed spinstep.continuous.QuaternionDepthIterator → spinstep.traversal.QuaternionDepthIterator
• Fixed QuaternionDepthIterator.__init__ signature (was missing rotation_step_quat, showed wrong params)

New usage guides

• docs/getting-started.md — install + first example
• docs/continuous-traversal.md — QuaternionDepthIterator usage, threshold behavior, edge cases
• docs/discrete-traversal.md — DiscreteOrientationSet factories + DiscreteQuaternionIterator
• docs/faq.md — pitfalls, GPU vs CPU, optional deps, quaternion format

Consistency fixes

• Fixed broken cross-reference links in docs/02-architecture.md (underscores → hyphens)
• Updated docs/index.md to link new guides

from spinstep import Node, QuaternionDepthIterator

root = Node("root", [0, 0, 0, 1], [
    Node("child", [0.2588, 0, 0, 0.9659])  # ~30° around Z
])

for node in QuaternionDepthIterator(root, [0.2588, 0, 0, 0.9659]):
    print(node.name)

No implementation logic changed. All 46 tests pass unchanged.

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