[llvm] [SandboxIR][Doc] Add a Doc file for Sandbox IR (PR #98691)

[Arthur Eubanks via llvm-commits]

================
@@ -288,3 +290,6 @@ Additional Topics
 
 :doc:`RISCV/RISCVVectorExtension`
    This document describes how the RISC-V Vector extension can be expressed in LLVM IR and how code is generated for it in the backend.
+
+:doc:`Sandbox IR <SandboxIR>`
+   Describes the design and usage of Sandbox IR, a transactional layer over LLVM IR.
----------------
aeubanks wrote:

```suggestion
   This document describes the design and usage of Sandbox IR, a transactional layer over LLVM IR.
```
(for consistency with above)

https://github.com/llvm/llvm-project/pull/98691