mle: make the lecture a sequel to fitting_distributions

- Compare the lognormal MLE with the method-of-moments fit, using the KS statistic
- Replace the by-eye tail comparison with log-scale Q-Q plots and KS
- Show why the method of moments fails for a Pareto tail with b <= 1
- Add a Student's t MLE fit to Amazon returns, as promised in fitting_distributions
- Fix the lognormal density, the sign of the derivative with respect to mu, and
  use \partial and aligned
- Add figure captions, axis labels and lw=2 per the style guide

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: jstac