[mlir][vector][doc] 0D vectors (#73792)

Currently, the only mention of 0d vectors in the MLIR source code that I
could find was:

>     0D vectors are allowed by omitting the dimension: `vector<f32>`.

in
[`BuiltinTypes.td`](https://github.com/llvm/llvm-project/blob/437a48b2d9bfc6e38a0ca43f1cee48ceaf0fe249/mlir/include/mlir/IR/BuiltinTypes.td#L1046-L1047).

This patch adds a summary of
https://discourse.llvm.org/t/what-is-the-semantics-of-memref-0xf32-and-tensor-0xf32/3557
and https://discourse.llvm.org/t/should-we-have-0-d-vectors/3097/5.