docs(style): Add Live, Interactive Instruction principle (#2927)

Adds a new pedagogical principle to STYLE.md to formalize the course's
core philosophy of live, interactive instruction.

This principle clarifies the expectation that instructors will interact
with code snippets live, use compiler errors as a teaching tool, and
engage with audience questions dynamically.

STYLE.md

@@ -46,6 +46,11 @@ We follow a few key principles to make the material effective for learning:
   usage syntax is a bit unusual, but we don't go into details of format strings
   or macros. We explain details of format strings later, once we have covered
   traits and can mention the `Debug` and `Display` traits.
+- **Live, Interactive Instruction:** The instructor is expected to run and
+  modify the code on the slides, and use compiler errors as a teaching tool. The
+  audience is expected to frequently interrupt with questions, and the
+  instructor would often experiment with the code on the slide to illustrate the
+  answer.
 
 ### Pacing and Structure