DOC: Clarify orientation transform semantics

[ColumbusLabs]

What

Clarify the direction and intended use of ornt_transform().

Why

Orientation arrays describe data axes relative to a shared output-axis system, typically RAS+. Making that direction explicit avoids reading an image's orientation as the inverse mapping.

Fixes #1418

Details

• defines start_ornt and end_ornt relative to the same output axes
• explains how the returned relative transform is applied
• adds an executable original-to-LPI example grounded in the issue discussion

Testing

• pytest -q nibabel/tests/test_orientations.py (8 passed, 1 xfailed)
• module doctests (4 passed)
• Sphinx doctest target (313 passed)
• Sphinx HTML build and rendered-output inspection
• full pre-commit suite
• git diff --check