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

[Kenneth Benzie via llvm-commits]

================
@@ -0,0 +1,21 @@
+.. Offload documentation master file, created by
+   sphinx-quickstart on Fri Jul  4 14:59:13 2025.
+   You can adapt this file completely to your liking, but it should at least
+   contain the root `toctree` directive.
----------------
kbenzie wrote:

This is the default `index.rst` from the `sphinx-quickstart` tool and I expect this to be changed in future with more relevant info. This PR is focused on introducing the Offload API docs.

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