[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 
+on `the Discourse forums (Clang Frontend category)
+<https://discourse.llvm.org/c/clang/6>`_ for wider visibility and open a 
+pull-request on `LLVM Github`_ to have it added here. Since the primary purpose of 
+this page is to provide examples that can help developers, the listed projects should
+have code available.
+
+As `clang-tidy` shares C++ AST Matchers with Clang diagnostics, `External Clang Examples`_ 
+may also be useful to look at for examples.
+
+.. _LLVM Github: https://github.com/llvm/llvm-project
+.. _External Clang Examples: https://clang.llvm.org/docs/ExternalClangExamples.html
+
+https://clang.llvm.org/docs/ExternalClangExamples.html
+
+List of projects and tools
+==========================
+
+`<https://github.com/coveooss/clang-tidy-plugin-examples/tree/main>`_
+    "This folder contains clang-tidy plugins."
----------------
MichelleCDjunaidi wrote:

followed ``index.rst`` conventions.

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