[clang-tools-extra] [doc] Add documentation for clang-change-namespace (PR #148277)

[Konrad Kleine via cfe-commits]

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+Clang-Change-Namespace
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kwk wrote:

@AaronBallman I'm open for this but I went with the way it's done by [`clang-include-fixer`](https://github.com/llvm/llvm-project/blob/9bf7d04c4386daf1ef0acf95782a59855c98474a/clang-tools-extra/docs/clang-include-fixer.rst#L2) and [`clang-tidy`](https://github.com/llvm/llvm-project/blob/7c402b8b81d2b69b55eb5bac39830fbc631f8cde/clang-tools-extra/docs/clang-tidy/index.rst#L2). Changing it would break with the convention over there.

https://github.com/llvm/llvm-project/pull/148277