[DOC] Update getting started site to explain CUDA 11.3 usage

[JohnZed]

We should add CUDA 11.3 to the release selector. This is a common request based on PyTorch's support for 11.3. It uses the 11.2-version packages, but with the cudatoolkit=11.3 specifier (has been tested on 21.12 and install works).

In the CUDA drivers/versions blurb above it on the getting started page, we should add a bullet for 11.3. We can call it experimental as it works perfectly fine per CEC, but we do not have it in CI matrix:

 11.2 & v460.27.03+
 **11.3 & v460.27.03+ (experimental)**
 11.4 & v470.42.01+