[Docs] Add docs to NVTX options

[yyccli]

📚 The doc issue

when building lmdeploy from build_all_wheel.sh, by default the NVTX option is ON at compile time. However, you also need to set environment variable FT_NVTX to ON so that the is_enable_ft_nvtx can be set to true and then nvtx push and nvtx pop are actually triggered.

Suggest a potential alternative/fix

Maybe just add some comments in entrypoint_build.sh or CMakeLists.txt to let users know : )