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

• Summarize the commit messages into a brief review of the Pull request.

[ianna]

@Saransh-cpp - I think, it would benefit from a little bit of explanation, but it's just me :-)

[ianna]

Perhaps, add a bit more details here?

The $(-,-,-,+)$ metric convention for Lorentz vectors corresponds to the Minkowski metric:

$g_{\mu\nu} = \text{diag}(-1, -1, -1, +1)$

For a Lorentz vector $p^\mu = (p_x, p_y, p_z, E)$, the squared norm (or invariant mass squared) is given by:

$p^\mu p_\mu = g_{\mu\nu} p^\mu p^\nu = E^2 - p_x^2 - p_y^2 - p_z^2$

This convention is widely used in high-energy physics (HEP), including frameworks such as ROOT.

[jonas-eschle]

I agree and would say this for the whole paragraph: if the Euler angle conventions align, why not write them out here too (copy-paste is even fine). Just to document it standalone. But useful addition anyways!

[Saransh-cpp]

Thanks for the review, @ianna and @jonas-eschle! I have added some more details in the paragraph. Let me know if it looks good! :)

[jonas-eschle]

Great, looks good to me, many thanks!

[Saransh-cpp]

Merging this in!