docs(indLin): restore the inductive-linearization material in the article

[mattfidler]

vignettes/articles/rxode2-ind-lin.Rmd is currently titled "Matrix Exponential
Solving in rxode2" and describes only what the solver actually does today.
All inductive-linearization material was deliberately removed rather than left
claiming behavior the solver does not have, and _pkgdown.yml:344 was retitled
to match.

Once the iteration is restored, put it back:

• the method itself (the y^[n] recursion and its convergence criterion), with
  the Duffull & Hegarty (2014), Hasegawa & Duffull (2018) and Sharif et al.
  (2022) references
• indLin(state) <- expr documented as the nonlinear forcing, explicitly
  noting that state(0) still sets initial conditions -- that distinction is
  the whole point of the syntax
• a re-measured hmax-versus-error table; the current default-hmax row
  (7.1e-01) stops being true once the iteration and the ODE-conversion change
  land
• restore the article title and the _pkgdown.yml nav text

Add NEWS.md entries per the format in CLAUDE.md, including the user-visible
change in results for nonlinear matExp() models.

Blocked by #1186.