Set up PlatformIO project for EL512.256-H on ESP32

PlatformIO skeleton (esp32dev, Arduino, Adafruit GFX) with a local EL512
library: EL512Panel (1 bpp framebuffer, two-bit-mode row encoder, I2S DMA
refresh stub) and EL512_GFX (Adafruit_GFX subclass). Adds a test sketch,
pin map, README with wiring and timing, and moves the datasheet to
docs/datasheet/.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.vscode/extensions.json

@@ -0,0 +1,11 @@
+{
+    // See http://go.microsoft.com/fwlink/?LinkId=827846
+    // for the documentation about the extensions.json format
+    "recommendations": [
+        "platformio.platformio-ide"
+    ],
+    "unwantedRecommendations": [
+        "ms-vscode.cpptools-extension-pack",
+        "pioarduino.pioarduino-ide"
+    ]
+}