[autodiff] Open with a sentence that suits both series

Programming and JAX both publish autodiff. John and Matt decided on
2026-10-12 that the two copies become one text (project-monorepo#111).
Programming's copy is that text, except its opening sentence, which
links to this series' own JAX preview and so cannot stand in JAX.

This changes only that sentence; the JAX series takes the same file.

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

Author: mmcky

lectures/autodiff.md

@@ -19,8 +19,8 @@ kernelspec:
 
 ## Overview
 
-This lecture gives a more thorough introduction to automatic differentiation
-using Google JAX, building on {doc}`our brief preview <jax_intro>`.
+This lecture gives an introduction to automatic differentiation using Google
+JAX.
 
 Automatic differentiation is one of the key elements of modern machine learning
 and artificial intelligence.