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>

.gitignore

@@ -0,0 +1,26 @@
+# PlatformIO
+.pio/
+.pioenvs/
+.piolibdeps/
+
+# VS Code (PlatformIO generates these per machine)
+.vscode/.browse.c_cpp.db*
+.vscode/c_cpp_properties.json
+.vscode/launch.json
+.vscode/ipch
+
+# Windows
+Thumbs.db
+ehthumbs.db
+Desktop.ini
+$RECYCLE.BIN/
+
+# macOS
+.DS_Store
+.AppleDouble
+.LSOverride
+._*
+
+# Editors
+*.swp
+*~