[SYCL][Doc] Add ExtendedAtomics extension (#1081)

* [SYCL][Doc] Add ExtendedAtomics extension

Adds missing atomic operators from C++11.
Adds floating-point support from C++20.

Signed-off-by: John Pennycook <[email protected]>

Author: Pennycook

sycl/doc/extensions/ExtendedAtomics/README.md

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+# SYCL_INTEL_extended_atomics
+
+Replaces the `cl::sycl::atomic` class with the `cl::sycl::intel::atomic` class, which exposes additional functionality aligned with the `std::atomic` class from C++11 and C++20.