[docs] Add a RISC-V Usage page

This follows the pattern set by e.g. SPIR-V, AMDGPU, and WebAssembly.

For the moment, this page is fairly minimal stub. I mostly want a place to record which extensions we support, and when that support is not complete, a central place to note that. I'm deliberately starting with a subset of extensions which are pretty straight forward with only one case needing a detail note.

I figure the partial support cases are going to need a bunch of discussion, and having each of them in their own review seemed worthwhile. Similarly, the experimental extensions probably fall into the same bucket of triggering a bunch of review discussion.

Differential Revision: https://reviews.llvm.org/D133063