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

[Kseniya Tikhomirova via llvm-commits]

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@@ -0,0 +1,52 @@
+========================
+Libsycl Coding Standards
+========================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+In general ``libsycl`` project adopts LLVM Coding Standards.
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KseniyaTikhomirova wrote:

done in https://github.com/llvm/llvm-project/pull/171867/commits/627fbf2606166b21653ac2402a281fcc916f7fdb.
I added external link, not relative link within the repo since there are two ways in RST to do that and their validity depends on generator. External link seems to be good for both of them.

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