Add quadrature encoder starter course

lessons/01-reading-encoder-signals/01-meet-the-quadrature-encoder/meet-the-quadrature-encoder.md

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+---
+title: Meet the Quadrature Encoder
+summary: Learn what the A and B channels are and why their phase offset lets you count motion.
+chapter: Reading Encoder Signals
+order: 1
+tags:
+  - encoders
+  - motors
+  - feedback
+estimated_minutes: 8
+---
+
+# Meet the Quadrature Encoder
+
+A quadrature encoder gives you two square-wave signals, usually called `A` and `B`.
+
+Those signals toggle as the shaft turns. The important detail is that they do not toggle at the same time. One channel leads the other by a quarter of a cycle, which is where the name quadrature comes from.
+
+That phase offset gives you two useful facts:
+
+1. You can count edges to measure motion.
+2. You can compare which signal leads to determine direction.
+
+## What you should observe
+
+- Slow rotation produces clean transitions you can inspect with a logic analyzer.
+- Reversing the shaft swaps which channel leads.
+- More pulses per revolution means finer position measurement.
+
+## Checkpoint
+
+Capture both channels while rotating the shaft by hand and confirm that one channel leads the other.