docs: add metric and transformation conventions

[Saransh-cpp]

Description

Add metric, transformation and boost, and Euler angle conventions followed by vector (aligned with the ones followed in ROOT and GenVector).

See openjournals/joss-reviews#7791 (comment)

Checklist

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't any other open Pull Requests for the required change?
• Does your submission pass pre-commit? ($ pre-commit run --all-files or $ nox -s lint)
• Does your submission pass tests? ($ pytest or $ nox -s tests)
• Does the documentation build with your changes? ($ cd docs; make clean; make html or $ nox -s docs)
• Does your submission pass the doctests? ($ pytest --doctest-plus src/vector/ or $ nox -s doctests)

Before Merging

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