[clang] [TySan] Add initial documentation for Type Sanitizer (PR #123595)

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+================
+TypeSanitizer
+================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+TypeSanitizer is a detector for strict type aliasing violations. It consists of a compiler
+instrumentation module and a run-time library. The tool detects violations where you access 
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gbMattN wrote:

I added some more text based off Florian's [original pitch](https://reviews.llvm.org/D32198)

https://github.com/llvm/llvm-project/pull/123595