[llvm] [SYCL] Add CodingGuidelines.rst for libsycl (PR #171867)

[Tom Honermann via llvm-commits]

https://github.com/tahonermann requested changes to this pull request.

Sorry for the long delay on this, @KseniyaTikhomirova. I posted a number of suggestions intended to simplify and shorten the document and to address grammar and spelling issues.

https://github.com/llvm/llvm-project/pull/171867