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>

platformio.ini

@@ -0,0 +1,12 @@
+; Planar EL512.256-H electroluminescent panel driver for ESP32 (ESP-WROOM-32)
+; https://docs.platformio.org/page/projectconf.html
+
+[env:esp32dev]
+platform = espressif32@6.10.0
+board = esp32dev
+framework = arduino
+monitor_speed = 115200
+lib_deps =
+    adafruit/Adafruit GFX Library
+build_flags =
+    -Wall