[Doc] We need to walk thru the Getting-started guide again

[xmhubj]

I run into some issues when following the getting started guide to setup using the lastest NGC container:

1. mis-matched link of example folder.
2. change directory to flagscale folder before running pip install.
3. Failed to pip install vllm-FL without change MAX_JOBS and TORCH_CUDA_ARCH_LIST
4. NGC's PyTorch container doesn't include conda, while the run script is using conda.

[aoyulong]

@xmhubj We will improve these guides once the installation process has been updated.