[llvm] [Offload] Add Offload API Sphinx documentation (PR #147323)

[Shilei Tian via llvm-commits]

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shiltian wrote:

A comment about general style: LLVM coding standard actually kind of discourages of using `auto`, unless its type is very obvious, like an iterator, or from a cast. I'd revisit all the `auto` uses in this file.

https://github.com/llvm/llvm-project/pull/147323