document χ^2 cut derivation

[slava77]

This is a multi-contribution issue,

• T5 rphi cuts are apparently from T5 regression and chi-squared based cuts, with pT5 as bonus (only the baseline selections) TrackLooper#68 This was by Balaji documented in 20210706-T5RegressionUpdate.pdf I'm guessing the full history is lost (per region plots are at a broken link likely lost with the UAF home accident).
• T5 r-z cuts are apparently introduced in Rebase t5 rz chi2 cut TrackLooper#281 . @YonsiG did you use some script to derive the values? If the python notebook or a script is around somewhere, it would be good to have it in the code (github gist or some repo would be OK). The pdf attached in that PR from 2023 apparently has the values that are in the PR now.