docs(book): added the docs.video2x.org mdBook source files and pipeline

Signed-off-by: k4yt3x <i@k4yt3x.com>

docs/book/src/running/command-line.md

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+# Command Line
+
+Instructions for running Video2X from the command line.
+
+This page does not cover all the options available. For help with more options available, run Video2X with the `--help` argument.
+
+## Basics
+
+Use the following command to upscale a video by 4x with RealESRGAN:
+
+```bash
+video2x -i input.mp4 -o output.mp4 -f realesrgan -r 4 -m realesr-animevideov3
+```
+
+Use the following command to upscale a video to with libplacebo + Anime4Kv4 Mode A+A:
+
+```bash
+video2x -i input.mp4 -o output.mp4 -f libplacebo -s anime4k-v4-a+a -w 3840 -h 2160
+```
+
+## Advanced
+
+It is possible to specify custom MPV-compatible GLSL shader files with the `--shader, -s` argument:
+
+```bash
+video2x -i input.mp4 -o output.mp4 -f libplacebo -s path/to/custom/shader.glsl -w 3840 -h 2160
+```
+
+List the available GPUs with `--list-gpus, -l`:
+
+```bash
+$video2x --list-gpus
+0. NVIDIA RTX A6000
+        Type: Discrete GPU
+        Vulkan API Version: 1.3.289
+        Driver Version: 565.228.64
+```
+
+Select which GPU to use with the `--gpu, -g` argument:
+
+```bash
+video2x -i input.mp4 -o output.mp4 -f realesrgan -r 4 -m realesr-animevideov3 -g 1
+```
+
+Specify arbitrary extra FFmepg encoder options with the `--extra-encoder-options, -e` argument:
+
+```bash
+video2x -i input.mkv -o output.mkv -f realesrgan -m realesrgan-plus -r 4 -c libx264rgb -e crf=17 -e preset=veryslow -e tune=film
+```