docs(esp32 std demo): Add README.md for esp32-std-demo

examples/README.md

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+# Mousefood Examples
+
+This directory contains the examples of `mousefood`.
+
+## ESP32 Examples
+
+- [ESP32 `std` Demo](./esp32-std-demo/README.md)
+- [ESP32 `no_std` Demo](./esp32-no-std-demo/README.md)
+
+## Waveshare Demo
+
+- [EPD Waveshare Demo](./epd-waveshare-demo/README.md)
+
+## Simulator
+
+- [Simulator Demo](./simulator/README.md)

examples/esp32-std-demo/README.md

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+# ESP32 `std` Demo
+
+Contains examples for `std` demo using [LILYGO T-Display](https://lilygo.cc/products/t-display).
+
+This demo showcases various UI widgets (like `Chart`, `Gauge`, and `Tabs`) while reading physical
+button presses and monitoring the board's battery voltage.
+
+## Pinmap
+
+![pinout](./assets/pin_diagram.jpg)
+
+### Essential Pins
+
+| Component    | Pin (GPIO) | Description            |
+| :----------- | :--------- | :--------------------- |
+| Display MOSI | 19         | SPI Data Out           |
+| Display SCLK | 18         | SPI Clock              |
+| Display CS   | 5          | Chip Select            |
+| Display DC   | 16         | Data/Command           |
+| Display RST  | 23         | Reset Pin              |
+| Display BL   | 4          | Backlight Control      |
+| Left Button  | 0          | User Input             |
+| Battery ADC  | 34         | Battery Voltage Reader |
+
+## Notes
+
+If you're adapting this demo for a different board or screen, there are some points to keep in mind:
+
+- **Screen Configuration:** The driver (`ST7789`), screen size
+  (`135x240`), and hardware offset (`52, 40`) are specific to the
+  T-Display panel. You will need to adjust these values in the code to
+  match your specific display.
+- **Battery Voltage Reader:** The T-Display PCB uses a voltage divider
+  that halves the battery voltage before it reaches the ADC on GPIO 34.
+  This is why the ADC reading is multiplied by 2 in the code. If you are
+  building a custom circuit, you will need to replicate this voltage
+  divider setup to safely measure battery levels.