[llvm] [Docs][RISCV] Document RISC-V vector codegen (PR #96740)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp commented:

I have written an internel doc about this before, and this is how I organized the content:
![图片](https://github.com/llvm/llvm-project/assets/137158460/ff0a548f-1dc1-45ca-abd2-311b087100ec)
Hopefully this can help us to perfect this doc. :-)


https://github.com/llvm/llvm-project/pull/96740