[clang-tools-extra] Update clang tidy Contributing guide (PR #106672)

[Julian Schmidt via cfe-commits]

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@@ -664,6 +679,17 @@ compiled against the version of clang-tidy that will be loading the plugin.
 The plugins can use threads, TLS, or any other facilities available to in-tree
 code which is accessible from the external headers.
 
+Note that testing checks out of tree might involve getting ``llvm-lit`` from an 
+installed version of LLVM through the `Stand-alone Builds`_ section. Alternatively,
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5chmidti wrote:

I'm not sure that we want to explicitly state `Stand-alone Builds` as the way to go. It would probably be better to say 
> from an LLVM installation compiled from source.

And link to the getting started page, in general. On the other hand, that would make this less specific


https://github.com/llvm/llvm-project/pull/106672