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

[Arthur Eubanks via llvm-commits]

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+# Sandbox IR: A transactional layer over LLVM IR
+
+Sandbox IR is an IR layer on top of LLVM IR that allows you to save/restore its state.
+
+# API
+The Sandbox IR API is designed to feel like LLVM, replicating many common API classes and functions to mirror the LLVM API.
+The class hierarchy is similar (but in the `llvm::sandboxir` namespace), for example here is a small part of it:
+```
+namespace sandboxir {
+              Value
+              /  \
+            User BasicBlock ...
+           /   \
+  Instruction Constant
+        /
+     ...
+}
+```
+
+# Design
+
+## Sandbox IR Value <-> LLVM IR Value Mapping
+Each LLVM IR Value maps to a single Sandbox IR Value.
+The reverse is also true in most cases, except for Sandbox IR Instructions that map to more than one LLVM IR Instruction.
+Such instructions can be defined in extensions of the base Sandbox IR.
+
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aeubanks wrote:

extra newline

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