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

[Danny Mösch via cfe-commits]

================
@@ -0,0 +1,32 @@
+============================
+External Clang-Tidy Examples
+============================
+
+Introduction
+============
+
+This page provides examples of what people have done with :program:`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 the `CMakeLists.txt`
+for an out-of-tree plugin of :program:`clang-tidy` checks.
+
+If you know of (or wrote!) a tool or project using :program:`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 :program:`clang-tidy` shares C++ AST Matchers with Clang diagnostics, 
+`External Clang Examples`_ may also be useful to look at for such 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
----------------
SimplyDanny wrote:

Is this some leftover that needs to be removed?

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