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

[Julian Schmidt 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 even 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 post on
+`the Discourse forums (Clang Frontend category)
+<https://discourse.llvm.org/c/clang/6>`_ to have it added.
+(or if you are already a clang-tidy contributor, feel free to directly commit
+additions). Since the primary purpose of this page is to provide examples
----------------
5chmidti wrote:

What do you think about:
> ..., please share it on <discourse> for wider visibility, and open a pull-request to have it added here. ~(or if you are already a clang-tidy contributor, feel free to directly commit additions)~

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