docs: add llvm version info to installation.md.

[SanftMonster]

When I built tile-lang from source, I found that compiling issues will raise if I use LLVM-20 due to the version of tvm submodule. Finally I fixed it by using lower version LLVM, therefore I'm adding a small hint to the doc in this PR.

Below is one of the errors:

/mnt/<user>/code/community/tilelang/3rdparty/tvm/src/target/llvm/codegen_amdgpu.cc:274:9: error: invalid use of incomplete type 'class llvm::Module'
  274 |     mlib->setTargetTriple(llvm_target->GetTargetTriple());

[LeiWang1999]

Thanks @AsakusaRinne , but this is strange because we’ve had llvm disabled for quite some time.

[SanftMonster]

@LeiWang1999 When I followed the instructions to build, I found that LLVM is used by setting the cmake option: https://github.com/tile-ai/tilelang/blob/main/docs/get_started/Installation.md?plain=1#L136. Does this DOC have been outdated?

[LeiWang1999]

yes @AsakusaRinne , thanks for your report, now de can set the USE_LLVM(OFF)