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

[Jorge Gorbe Moya via llvm-commits]

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@@ -0,0 +1,52 @@
+# Sandbox IR: A transactional layer over LLVM IR
+
+Sandbox IR is an IR layer on top of LLVM IR that allow you to save/restore its state.
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slackito wrote:

```suggestion
Sandbox IR is an IR layer on top of LLVM IR that allows you to save/restore its state.
```

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