[llvm] [compiler-rt] [docs][IRPGO]Document two binary formats for IRPGO profiles (PR #76105)

[Fangrui Song via llvm-commits]

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MaskRay wrote:

Perhaps name this ProfileFormat.rst, since so much is shared with pure code coverage applications. It's fine that the doc currently focuses on PGO. 

https://github.com/llvm/llvm-project/pull/76105