convert lvgl from submodule to a plain old directory

lib/lvgl/docs/libs/ffmpeg.md

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+
+# FFmpeg support
+[FFmpeg](https://www.ffmpeg.org/) A complete, cross-platform solution to record, convert and stream audio and video.
+
+## Install FFmpeg
+- Download FFmpeg from [here](https://www.ffmpeg.org/download.html)
+- `./configure --disable-all --disable-autodetect --disable-podpages --disable-asm --enable-avcodec --enable-avformat --enable-decoders --enable-encoders --enable-demuxers --enable-parsers --enable-protocol='file' --enable-swscale --enable-zlib`
+- `make`
+- `sudo make install`
+
+## Add FFmpeg to your project
+- Add library: `FFmpeg` (for GCC: `-lavformat -lavcodec -lavutil -lswscale -lm -lz -lpthread`)
+
+## Usage
+
+Enable `LV_USE_FFMPEG` in `lv_conf.h`.
+
+See the examples below.
+
+Note that, the FFmpeg extension doesn't use LVGL's file system.
+You can simply pass the path to the image or video as usual on your operating system or platform.
+
+## Example
+```eval_rst
+
+.. include:: ../../examples/libs/ffmpeg/index.rst
+
+```
+
+## API
+
+```eval_rst
+
+.. doxygenfile:: lv_ffmpeg.h
+  :project: lvgl
+
+```