README section for video backends

[bjuncek]

📚 Documentation

Describing the video backends, APIs, installation and requirements in the main README.md.
Specifically targeted to address issues such as #3367 #3440 and have a clearer reference point for the future users.

cc @bjuncek

[Tenglon]

VideoReader building documentation is needed.

I am building from the source in the following order (Nvdia-driver, CUDA 11.4, build pytorch from source, build ffmpeg with GPU support from source and build torchvision from source), but it still says that I need to build from source to use VideoReader.