[clang-tools-extra] Add clang-tidy external examples (PR #106675)

[via cfe-commits]

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+=======================
+External Clang-Tidy Examples
+=======================
+
+Introduction
+============
+
+This page provides examples of what people have done with `clang-tidy` that 
+might serve as useful guides (or starting points) to develop your own checks. 
+They may be helpful for necessary things such as how to write CMakeLists.txt 
+for an out-of-tree plugin of `clang-tidy` checks.
+
+If you know of (or wrote!) a tool or project using clang-tidy, please share it 
----------------
MichelleCDjunaidi wrote:

actually, a question: why not use :program:`clang-tidy`?

https://github.com/llvm/llvm-project/pull/106675