[Docs] Outdated installation documentation

[Mr-Brainiac237]

I've been using TVM for research and when I was installing, I ran into an issue with installation. Based on the installation docs for Install from Source, there should be a _ffi folder created during installation that would house the C++ bindings. However, upon further inspection, it seems that a _ffi folder doesn't exist in the current version of TVM. This then means that the current validation of
python -c "import tvm; print(tvm._ffi.base._LIB)"
is incorrect and misleading. I really do hope that the c++ wrappers and extensions are still part of this project. However, it seems an update to docs are heavily needed.

[w1049]

According to commit 4289efa, tvm._ffi has been phased out. After this commit, it appears that tvm.ffi should be used instead.

[tqchen]

great, catch, the docs should be updated by #18056

[yongwww]

It should be resolved now. @Mr-Brainiac237