Create gray_scott.md

[sharvmurgai]

Tutorial for Learning Non-Linear Reaction Dynamics for the Gray–Scott Reaction–Diffusion Model using Universal Differential Equations

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[ChrisRackauckas]

time span is too short. Change it to something like 0 to 5 and make a forward plot. What does it look like?

[ChrisRackauckas]

│   exception =
--
  | │    UndefVarError: `N` not defined
  | │    Stacktrace:
  | │      [1] top-level scope
  | │        @ gray_scott.md:350