docs(count-regression): fix semantic line breaks and improve math derivations

Fix mid-line semicolon line break check in deviance residuals definition
and improve narrative flow and mathematical derivations in count regression chapter:
- Split deviance residual display equation across semantic line breaks.
- Annotate each step in mathematical derivations with parenthetical
  explanations across all count regression subfiles.
- Standardize estimator macros (\eb_j, \ey, \emu, \el, \eeta) and vector
  transpose notation (\tp{\vx}\vb).
- Disambiguate null parameter value (\beta_{j,0}) from intercept (\beta_0)
  in Wald test descriptions.
- Improve narrative transitions and replace bare demonstrative pronouns.

Co-authored-by: dem-extra1 <112029334+dem-extra1@users.noreply.github.com>

Author: google-labs-jules[bot]